feat: Dashboard – Donut ohne Überlauf, Achse lesbar, Karten-Optionen

- Budget-Donut: Mittelbetrag passt sich der Länge an (kein Hineinlaufen in den
  Ring bei 4-stelligen Beträgen)
- Vermögensverlauf: Y-Achse ohne Währungssymbol (formatAmount) + etwas mehr
  linker Rand → „CHF …" wird nicht mehr abgeschnitten
- Karten-Optionen: im Anordnen-Modus je Karte einstellbar; Budget-Donut und
  Budget-Fortschritt können nach Gruppen ODER Kategorien aufschlüsseln
  (?group_by=), Auswahl wird im Layout (props) mitgespeichert

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

backend/app/routers/dashboard.py

@@ -99,47 +99,69 @@ def _expense_groups(db: Session, user: User) -> list[CategoryGroup]:
     )
 
 
+def _units(groups: list[CategoryGroup], by_category: bool):
+    """Liefert je Anzeige-Einheit (name, farbe, [kategorie-ids]). Bei ``by_category``
+    ist jede Kategorie eine eigene Einheit, sonst jede Gruppe (Summe ihrer Kategorien)."""
+    for g in groups:
+        if by_category:
+            for c in g.categories:
+                if not c.is_archived:
+                    yield (c.name, c.color, [c.id])
+        else:
+            yield (g.name, g.color, [c.id for c in g.categories if not c.is_archived])
+
+
 @router.get("/budget-donut")
-def budget_donut(db: Session = Depends(get_db), user: User = Depends(get_current_user)) -> dict:
+def budget_donut(
+    group_by: str = Query(default="group"),
+    db: Session = Depends(get_db),
+    user: User = Depends(get_current_user),
+) -> dict:
     base = base_currency(db, user)
     label = _label(db, user)
     start, end = budget.period_bounds(label, _start_day(db, user))
     planned = budget.effective_planned(db, user, label)
     groups = _expense_groups(db, user)
+    by_category = group_by == "category"
 
-    segments = []
-    total = 0
-    for g in groups:
-        val = sum(planned.get(c.id, 0) for c in g.categories if not c.is_archived)
-        if val > 0:
-            segments.append({"name": g.name, "value": val, "color": g.color})
-            total += val
+    def collect(values: dict[int, int]):
+        segs, tot = [], 0
+        for name, color, ids in _units(groups, by_category):
+            val = sum(values.get(i, 0) for i in ids)
+            if val > 0:
+                segs.append({"name": name, "value": val, "color": color})
+                tot += val
+        return segs, tot
+
+    segments, total = collect(planned)
     mode = "planned"
     if total == 0:
-        actual = budget.actual_expense_by_category(db, user, start, end, base)
-        for g in groups:
-            val = sum(actual.get(c.id, 0) for c in g.categories if not c.is_archived)
-            if val > 0:
-                segments.append({"name": g.name, "value": val, "color": g.color})
-                total += val
+        segments, total = collect(budget.actual_expense_by_category(db, user, start, end, base))
         mode = "actual"
-    return {"segments": segments, "total": total, "currency": base, "mode": mode, "label": label}
+    return {"segments": segments, "total": total, "currency": base, "mode": mode,
+            "label": label, "group_by": "category" if by_category else "group"}
 
 
 @router.get("/budget-progress")
-def budget_progress(db: Session = Depends(get_db), user: User = Depends(get_current_user)) -> dict:
+def budget_progress(
+    group_by: str = Query(default="group"),
+    db: Session = Depends(get_db),
+    user: User = Depends(get_current_user),
+) -> dict:
     base = base_currency(db, user)
     label = _label(db, user)
     start, end = budget.period_bounds(label, _start_day(db, user))
     planned = budget.effective_planned(db, user, label)
     actual = budget.actual_expense_by_category(db, user, start, end, base)
+    by_category = group_by == "category"
     rows = []
-    for g in _expense_groups(db, user):
-        gp = sum(planned.get(c.id, 0) for c in g.categories if not c.is_archived)
-        ga = sum(actual.get(c.id, 0) for c in g.categories if not c.is_archived)
+    for name, color, ids in _units(_expense_groups(db, user), by_category):
+        gp = sum(planned.get(i, 0) for i in ids)
+        ga = sum(actual.get(i, 0) for i in ids)
         if gp > 0 or ga > 0:
-            rows.append({"name": g.name, "color": g.color, "planned": gp, "actual": ga, "over": ga > gp})
-    return {"rows": rows, "currency": base, "label": label}
+            rows.append({"name": name, "color": color, "planned": gp, "actual": ga, "over": ga > gp})
+    return {"rows": rows, "currency": base, "label": label,
+            "group_by": "category" if by_category else "group"}
 
 
 @router.get("/income-expense")