feat: Budget – mehrere Monate nebeneinander + Kopieren Monat<->Vorlage

- Umschalter 1·2·3: Monate nebeneinander (responsives Raster, auf schmalem
  Schirm gestapelt); prev/next verschiebt das ganze Fenster
- Je Monat: „Als Standard festlegen" (Monat -> Vorlage) und „Standard
  übernehmen" (Vorlage -> Monat, überschreibt), plus Zurücksetzen
- Backend: copy_month_to_template / copy_template_to_month + Endpunkte
  POST /budget/template/from-month/{label} und /budget/{label}/from-template
- Test fürs Kopieren; globale .grow-Utility ergänzt

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/pages/Budget.jsx

@@ -20,6 +20,13 @@ function ProgressBar({ actual, planned }) {
   );
 }
 
+// "YYYY-MM" um n Monate verschieben (rein clientseitig, für die Nebeneinander-Ansicht).
+function shiftLabel(label, n) {
+  const [y, m] = label.split("-").map(Number);
+  const d = new Date(y, m - 1 + n, 1);
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}`;
+}
+
 export default function Budget() {
   const { t } = useTranslation();
   const { locale } = useSettings();
@@ -27,21 +34,24 @@ export default function Budget() {
   const toast = useToast();
 
   const [mode, setMode] = useState("month"); // "month" | "template"
-  const [view, setView] = useState(null);
-  const [label, setLabel] = useState(null);
+  const [views, setViews] = useState(null);  // Array von Monatsansichten (1..3)
+  const [anchor, setAnchor] = useState(null); // Label des ersten (linken) Monats
+  const [count, setCount] = useState(1);
   const [groups, setGroups] = useState([]);
   const [accounts, setAccounts] = useState([]);
   const [template, setTemplate] = useState({ lines: [], income: [] });
   const [incomeModal, setIncomeModal] = useState(null);
 
-  const base = view?.base_currency || "EUR";
+  const base = views?.[0]?.base_currency || "EUR";
   const fmt = (c) => formatMoney(c, base);
 
-  async function loadView(lbl) {
+  async function loadViews(anchorLbl, n) {
     try {
-      const v = lbl ? await api.budget(lbl) : await api.budgetCurrent();
-      setView(v);
-      setLabel(v.label);
+      const first = anchorLbl ? await api.budget(anchorLbl) : await api.budgetCurrent();
+      const rest = [];
+      for (let k = 1; k < n; k++) rest.push(await api.budget(shiftLabel(first.label, k)));
+      setViews([first, ...rest]);
+      setAnchor(first.label);
     } catch (err) { toast(err.message, "error"); }
   }
   async function loadTemplate() {
@@ -54,7 +64,7 @@ export default function Budget() {
     } catch (err) { toast(err.message, "error"); }
   }
 
-  useEffect(() => { loadStamm(); loadView(); }, []);
+  useEffect(() => { loadStamm(); loadViews(null, 1); }, []);
   useEffect(() => { if (mode === "template") loadTemplate(); }, [mode]);
 
   const tplMap = useMemo(
@@ -67,16 +77,32 @@ export default function Budget() {
     [groups]
   );
 
-  async function saveMonth(catId, cents) {
-    try { setView(await api.setMonthCategory(label, catId, cents)); } catch (err) { toast(err.message, "error"); }
+  function replaceView(v) {
+    setViews((vs) => vs.map((x) => (x.label === v.label ? v : x)));
   }
+  function changeCount(n) { setCount(n); loadViews(anchor, n); }
+
+  async function saveMonth(lbl, catId, cents) {
+    try { replaceView(await api.setMonthCategory(lbl, catId, cents)); } catch (err) { toast(err.message, "error"); }
+  }
+  async function resetMonth(lbl) {
+    if (!(await confirm({ title: t("budget.resetTitle"), message: t("budget.resetConfirm"), confirmLabel: t("budget.reset") }))) return;
+    try { replaceView(await api.resetMonth(lbl)); toast(t("settings.saved")); } catch (err) { toast(err.message, "error"); }
+  }
+  async function copyMonthToTemplate(lbl) {
+    if (!(await confirm({ title: t("budget.setAsTemplate"), message: t("budget.setAsTemplateConfirm"), confirmLabel: t("budget.setAsTemplate") }))) return;
+    try { await api.budgetTemplateFromMonth(lbl); toast(t("settings.saved")); if (mode === "template") loadTemplate(); }
+    catch (err) { toast(err.message, "error"); }
+  }
+  async function copyTemplateToMonth(lbl) {
+    if (!(await confirm({ title: t("budget.copyFromTemplate"), message: t("budget.copyFromTemplateConfirm"), confirmLabel: t("budget.copyFromTemplate"), danger: true }))) return;
+    try { replaceView(await api.budgetMonthFromTemplate(lbl)); toast(t("settings.saved")); }
+    catch (err) { toast(err.message, "error"); }
+  }
+
   async function saveTemplate(catId, cents) {
     try { await api.setTemplateCategory(catId, cents); loadTemplate(); } catch (err) { toast(err.message, "error"); }
   }
-  async function resetMonth() {
-    if (!(await confirm({ title: t("budget.resetTitle"), message: t("budget.resetConfirm"), confirmLabel: t("budget.reset") }))) return;
-    try { setView(await api.resetMonth(label)); toast(t("settings.saved")); } catch (err) { toast(err.message, "error"); }
-  }
 
   async function saveIncome(e) {
     e.preventDefault();
@@ -92,17 +118,77 @@ export default function Budget() {
       else await api.createIncome(body);
       setIncomeModal(null);
       loadTemplate();
-      loadView(label);
+      loadViews(anchor, count);
     } catch (err) { toast(err.message, "error"); }
   }
   async function deleteIncome(id) {
     if (!(await confirm({ title: t("common.delete"), message: t("budget.deleteIncomeConfirm"), danger: true, confirmLabel: t("common.delete") }))) return;
-    try { await api.deleteIncome(id); loadTemplate(); loadView(label); } catch (err) { toast(err.message, "error"); }
+    try { await api.deleteIncome(id); loadTemplate(); loadViews(anchor, count); } catch (err) { toast(err.message, "error"); }
   }
 
-  if (!view) return <div className="muted">{t("common.loading")}</div>;
+  function MonthColumn({ view: v }) {
+    const totv = v.totals;
+    return (
+      <div className="card budget-col">
+        <div className="budget-col-head">
+          <strong className="grow ellipsis">{formatMonthLabel(v.label, locale)}</strong>
+          <button className="btn sm ghost" title={t("budget.copyFromTemplate")} onClick={() => copyTemplateToMonth(v.label)}>
+            <Icon name="download" size={14} />
+          </button>
+          <button className="btn sm ghost" title={t("budget.setAsTemplate")} onClick={() => copyMonthToTemplate(v.label)}>
+            <Icon name="upload" size={14} />
+          </button>
+          {v.customized && (
+            <button className="btn sm ghost" title={t("budget.reset")} onClick={() => resetMonth(v.label)}>
+              <Icon name="refresh-ccw" size={14} />
+            </button>
+          )}
+        </div>
 
-  const tot = view.totals;
+        {v.income.length > 0 && (
+          <div className="budget-col-income">
+            <span className="grow muted small">{t("budget.income")}</span>
+            <span className={`amount small ${totv.received_income >= totv.planned_income ? "pos" : "neg"}`}>{fmt(totv.received_income)}</span>
+            <span className="muted small"> / {fmt(totv.planned_income)}</span>
+          </div>
+        )}
+
+        {v.groups.map((g) => (
+          <div className="budget-group" key={g.id}>
+            <div className="budget-group-head">
+              <IconChip name={g.icon} color={g.color} size={22} />
+              <strong className="grow ellipsis">{g.name}</strong>
+              <span className={`amount small ${g.over ? "neg" : "muted"}`}>{fmt(g.actual_cents)}</span>
+              <span className="muted small"> / {fmt(g.planned_cents)}</span>
+            </div>
+            <ProgressBar actual={g.actual_cents} planned={g.planned_cents} />
+            {g.categories.map((c) => (
+              <div className="budget-crow" key={c.category_id}>
+                <IconChip name={c.icon} color={c.color} size={20} />
+                <span className="grow ellipsis">{c.name}</span>
+                <span className={`amount small ${c.over ? "neg" : "muted"}`}>{fmt(c.actual_cents)}</span>
+                <EditableAmount cents={c.planned_cents} onSave={(cents) => saveMonth(v.label, c.category_id, cents)} />
+              </div>
+            ))}
+          </div>
+        ))}
+
+        <div className="budget-total-row card-total">
+          <span className="grow"><strong>{t("budget.totalExpense")}</strong></span>
+          <span className={`amount ${totv.actual_expense > totv.planned_expense ? "neg" : ""}`}>{fmt(totv.actual_expense)}</span>
+          <span className="muted"> / {fmt(totv.planned_expense)}</span>
+        </div>
+      </div>
+    );
+  }
+
+  if (!views) return <div className="muted">{t("common.loading")}</div>;
+
+  const first = views[0]?.label;
+  const last = views[views.length - 1]?.label;
+  const rangeTitle = views.length === 1
+    ? formatMonthLabel(first, locale)
+    : `${formatMonthLabel(first, locale)} – ${formatMonthLabel(last, locale)}`;
 
   return (
     <div>
@@ -121,76 +207,24 @@ export default function Budget() {
       {mode === "month" ? (
         <>
           <div className="card budget-navbar">
-            <button className="btn ghost" onClick={() => loadView(view.prev)}>
+            <button className="btn ghost" onClick={() => loadViews(shiftLabel(anchor, -1), count)} title={t("budget.prevMonth")}>
               <Icon name="chevron-right" size={16} className="flip" />
             </button>
-            <strong className="budget-month">{formatMonthLabel(view.label, locale)}</strong>
-            <button className="btn ghost" onClick={() => loadView(view.next)}>
+            <strong className="budget-month">{rangeTitle}</strong>
+            <button className="btn ghost" onClick={() => loadViews(shiftLabel(anchor, 1), count)} title={t("budget.nextMonth")}>
               <Icon name="chevron-right" size={16} />
             </button>
             <div className="grow" />
-            {view.customized && (
-              <button className="btn sm" onClick={resetMonth}>
-                <Icon name="refresh-ccw" size={14} /> {t("budget.reset")}
-              </button>
-            )}
+            <span className="muted small">{t("budget.monthsShown")}</span>
+            <div className="seg sm" style={{ width: "auto" }}>
+              {[1, 2, 3].map((n) => (
+                <button key={n} type="button" className={`seg-btn${count === n ? " active" : ""}`} onClick={() => changeCount(n)}>{n}</button>
+              ))}
+            </div>
           </div>
 
-          {view.income.length > 0 && (
-            <div className="card">
-              <h2>{t("budget.income")}</h2>
-              <table className="table">
-                <tbody>
-                  {view.income.map((i) => (
-                    <tr key={i.id}>
-                      <td>{i.label}</td>
-                      <td style={{ textAlign: "right" }} className="muted">{t("budget.planned")}: {fmt(i.planned_cents)}</td>
-                      <td style={{ textAlign: "right" }}>
-                        <span className={`amount ${i.received_cents >= i.planned_cents ? "pos" : "neg"}`}>{fmt(i.received_cents)}</span>
-                      </td>
-                    </tr>
-                  ))}
-                  <tr className="budget-total-row">
-                    <td><strong>{t("budget.totalIncome")}</strong></td>
-                    <td style={{ textAlign: "right" }} className="muted">{fmt(tot.planned_income)}</td>
-                    <td style={{ textAlign: "right" }}><strong>{fmt(tot.received_income)}</strong></td>
-                  </tr>
-                </tbody>
-              </table>
-            </div>
-          )}
-
-          <div className="card">
-            <h2>{t("budget.expenses")}</h2>
-            {view.groups.map((g) => (
-              <div className="budget-group" key={g.id}>
-                <div className="budget-group-head">
-                  <IconChip name={g.icon} color={g.color} size={26} />
-                  <span className="grow"><strong>{g.name}</strong></span>
-                  <span className={`amount ${g.over ? "neg" : "muted"}`}>{fmt(g.actual_cents)}</span>
-                  <span className="muted"> / {fmt(g.planned_cents)}</span>
-                </div>
-                {g.categories.map((c) => (
-                  <div className="budget-cat" key={c.category_id}>
-                    <div className="budget-cat-name">
-                      <IconChip name={c.icon} color={c.color} size={24} />
-                      <span>{c.name}</span>
-                    </div>
-                    <div className="budget-cat-bar">
-                      <ProgressBar actual={c.actual_cents} planned={c.planned_cents} />
-                    </div>
-                    <span className={`amount budget-actual ${c.over ? "neg" : "muted"}`}>{fmt(c.actual_cents)}</span>
-                    <span className="muted budget-slash">/</span>
-                    <EditableAmount cents={c.planned_cents} onSave={(cents) => saveMonth(c.category_id, cents)} />
-                  </div>
-                ))}
-              </div>
-            ))}
-            <div className="budget-total-row card-total">
-              <span className="grow"><strong>{t("budget.totalExpense")}</strong></span>
-              <span className={`amount ${tot.actual_expense > tot.planned_expense ? "neg" : ""}`}>{fmt(tot.actual_expense)}</span>
-              <span className="muted"> / {fmt(tot.planned_expense)}</span>
-            </div>
+          <div className="budget-months" style={{ "--cols": views.length }}>
+            {views.map((v) => <MonthColumn key={v.label} view={v} />)}
           </div>
         </>
       ) : (