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/zh.json

@@ -77,7 +77,14 @@
     "dueSoon": "今天到期",
     "dueTomorrow": "明天到期",
     "allDay": "全天",
-    "shoppingMore": "+{{count}} 更多"
+    "shoppingMore": "+{{count}} 更多",
+    "weather": "天气",
+    "customize": "自定义",
+    "customizeTitle": "自定义小组件",
+    "customizeReset": "重置",
+    "customizeSaved": "仪表板已保存",
+    "customizeMoveUp": "上移",
+    "customizeMoveDown": "下移"
   },
   "tasks": {
     "title": "任务",
@@ -589,4 +596,4 @@
     "unitMonth": "个月",
     "unitMonths": "个月"
   }
-}
+}