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/locales/en.json

@@ -236,6 +236,13 @@
     "resetTitle": "Reset month",
     "resetConfirm": "Reset this month to the standard budget? This month's adjustments will be lost.",
     "templateHint": "The standard budget applies to every month until you adjust a month individually.",
+    "prevMonth": "Previous month",
+    "nextMonth": "Next month",
+    "monthsShown": "Months:",
+    "setAsTemplate": "Set as standard",
+    "setAsTemplateConfirm": "Use this month's amounts as the new standard budget? The current template is replaced (planned income stays).",
+    "copyFromTemplate": "Apply standard",
+    "copyFromTemplateConfirm": "Overwrite this month with the standard budget? This month's adjustments will be lost.",
     "addIncome": "Planned income",
     "editIncome": "Edit income",
     "noIncome": "No planned income yet.",