feat(06-02): replace BudgetSetup with template-based month picker

- Rewrite BudgetSetup to use month picker + currency + Generate button
- Remove manual form fields (name, dates, carryover, copy-from select)
- Handle 409 conflict gracefully by calling onCreated() to refresh list
- Remove copyFrom method from budgets API (TMPL-06)
- Update BudgetSetup test to reflect new month-picker UI
- Remove copyFrom from DashboardPage test mock
- Add budget.generate, budget.month, budget.generating i18n keys (EN/DE)
- Remove budget.copyFrom and budget.setup i18n keys

frontend/src/i18n/de.json

@@ -49,9 +49,10 @@
     "endDate": "Enddatum",
     "currency": "Waehrung",
     "carryover": "Uebertrag",
-    "copyFrom": "Vom vorherigen kopieren",
     "selectBudget": "Budget auswaehlen",
-    "setup": "Einrichtung"
+    "generate": "Aus Vorlage erstellen",
+    "month": "Monat",
+    "generating": "Wird erstellt..."
   },
   "category": {
     "create": "Kategorie erstellen",

frontend/src/i18n/en.json

@@ -49,9 +49,10 @@
     "endDate": "End Date",
     "currency": "Currency",
     "carryover": "Carryover",
-    "copyFrom": "Copy from previous",
     "selectBudget": "Select Budget",
-    "setup": "Setup"
+    "generate": "Generate from Template",
+    "month": "Month",
+    "generating": "Generating..."
   },
   "category": {
     "create": "Create Category",