feat(04-03): upgrade BudgetListPage with PageShell, locale-aware months, skeleton, and i18n labels

- Replace hardcoded MONTHS array with Intl.DateTimeFormat locale-aware monthItems useMemo
- Update budgetLabel helper to accept locale parameter and use Intl.DateTimeFormat
- Replace null loading state with PageShell + Skeleton rows
- Replace manual header with PageShell title and action prop
- Replace hardcoded 'Month'/'Year' Labels with t('budgets.month')/t('budgets.year')
- Add month/year/total keys to en.json and de.json budgets namespace

src/i18n/de.json

@@ -61,7 +61,10 @@
     "notes": "Notizen",
     "addItem": "Eintrag hinzufügen",
     "empty": "Noch keine Budgets. Erstelle dein erstes Monatsbudget.",
-    "deleteConfirm": "Bist du sicher, dass du dieses Budget löschen möchtest?"
+    "deleteConfirm": "Bist du sicher, dass du dieses Budget löschen möchtest?",
+    "month": "Monat",
+    "year": "Jahr",
+    "total": "{{label}} Gesamt"
   },
   "dashboard": {
     "title": "Dashboard",

src/i18n/en.json

@@ -61,7 +61,10 @@
     "notes": "Notes",
     "addItem": "Add Item",
     "empty": "No budgets yet. Create your first monthly budget.",
-    "deleteConfirm": "Are you sure you want to delete this budget?"
+    "deleteConfirm": "Are you sure you want to delete this budget?",
+    "month": "Month",
+    "year": "Year",
+    "total": "{{label}} Total"
   },
   "dashboard": {
     "title": "Dashboard",