feat(todo): add To-Do list feature with 3-column layout

- New todo_items DB table with priority, due date, description, user assignment
- Full CRUD API with WebSocket real-time sync
- 3-column UI: sidebar filters (All, My Tasks, Overdue, Done, by Priority),
  task list with inline badges, and detail/create pane
- Apple-inspired design with custom dropdowns, date picker, priority system (P1-P3)
- Mobile responsive: icon-only sidebar, bottom-sheet modals for detail/create
- Lists tab with sub-tabs (Packing List + To-Do), persisted selection
- Addon renamed from "Packing List" to "Lists"
- i18n keys for all 13 languages
- UI polish: notification colors use system theme, mobile navbar cleanup,
  settings page responsive buttons

client/src/types.ts

@@ -86,6 +86,19 @@ export interface PackingItem {
   quantity: number
 }
 
+export interface TodoItem {
+  id: number
+  trip_id: number
+  name: string
+  category: string | null
+  checked: number
+  sort_order: number
+  due_date: string | null
+  description: string | null
+  assigned_user_id: number | null
+  priority: number
+}
+
 export interface Tag {
   id: number
   name: string