feat: customizable dashboard layout (#32)

Users can now show/hide widgets and reorder them via a settings button
in the greeting header. Configuration is persisted server-side in
sync_config (dashboard_widgets key) and shared across all family members.

- Greeting widget gets a settings icon button opening a customize modal
- Modal lists all widgets (tasks, calendar, shopping, meals, notes,
  weather) with toggle switches and up/down reorder buttons
- Reset to default layout available in the modal
- GET /preferences now returns dashboard_widgets; PUT accepts it
- All 10 locales updated with new i18n keys

public/locales/de.json

@@ -77,7 +77,14 @@
     "dueSoon": "Heute fällig",
     "dueTomorrow": "Morgen fällig",
     "allDay": "Ganztägig",
-    "shoppingMore": "+{{count}} weitere"
+    "shoppingMore": "+{{count}} weitere",
+    "weather": "Wetter",
+    "customize": "Anpassen",
+    "customizeTitle": "Widgets anpassen",
+    "customizeReset": "Standard",
+    "customizeSaved": "Dashboard gespeichert",
+    "customizeMoveUp": "Nach oben",
+    "customizeMoveDown": "Nach unten"
   },
   "tasks": {
     "title": "Aufgaben",