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/store/slices/remoteEventHandler.ts

@@ -1,6 +1,6 @@
 import type { StoreApi } from 'zustand'
 import type { TripStoreState } from '../tripStore'
-import type { Assignment, Place, Day, DayNote, PackingItem, BudgetItem, BudgetMember, Reservation, Trip, TripFile, WebSocketEvent } from '../../types'
+import type { Assignment, Place, Day, DayNote, PackingItem, TodoItem, BudgetItem, BudgetMember, Reservation, Trip, TripFile, WebSocketEvent } from '../../types'
 
 type SetState = StoreApi<TripStoreState>['setState']
 
@@ -175,6 +175,19 @@ export function handleRemoteEvent(set: SetState, event: WebSocketEvent): void {
           packingItems: state.packingItems.filter(i => i.id !== payload.itemId),
         }
 
+      // Todo
+      case 'todo:created':
+        if (state.todoItems.some(i => i.id === (payload.item as TodoItem).id)) return {}
+        return { todoItems: [...state.todoItems, payload.item as TodoItem] }
+      case 'todo:updated':
+        return {
+          todoItems: state.todoItems.map(i => i.id === (payload.item as TodoItem).id ? payload.item as TodoItem : i),
+        }
+      case 'todo:deleted':
+        return {
+          todoItems: state.todoItems.filter(i => i.id !== payload.itemId),
+        }
+
       // Budget
       case 'budget:created':
         if (state.budgetItems.some(i => i.id === (payload.item as BudgetItem).id)) return {}