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/todoSlice.ts

@@ -0,0 +1,67 @@
+import { todoApi } from '../../api/client'
+import type { StoreApi } from 'zustand'
+import type { TripStoreState } from '../tripStore'
+import type { TodoItem } from '../../types'
+import { getApiErrorMessage } from '../../types'
+
+type SetState = StoreApi<TripStoreState>['setState']
+type GetState = StoreApi<TripStoreState>['getState']
+
+export interface TodoSlice {
+  addTodoItem: (tripId: number | string, data: Partial<TodoItem>) => Promise<TodoItem>
+  updateTodoItem: (tripId: number | string, id: number, data: Partial<TodoItem>) => Promise<TodoItem>
+  deleteTodoItem: (tripId: number | string, id: number) => Promise<void>
+  toggleTodoItem: (tripId: number | string, id: number, checked: boolean) => Promise<void>
+}
+
+export const createTodoSlice = (set: SetState, get: GetState): TodoSlice => ({
+  addTodoItem: async (tripId, data) => {
+    try {
+      const result = await todoApi.create(tripId, data)
+      set(state => ({ todoItems: [...state.todoItems, result.item] }))
+      return result.item
+    } catch (err: unknown) {
+      throw new Error(getApiErrorMessage(err, 'Error adding todo'))
+    }
+  },
+
+  updateTodoItem: async (tripId, id, data) => {
+    try {
+      const result = await todoApi.update(tripId, id, data)
+      set(state => ({
+        todoItems: state.todoItems.map(item => item.id === id ? result.item : item)
+      }))
+      return result.item
+    } catch (err: unknown) {
+      throw new Error(getApiErrorMessage(err, 'Error updating todo'))
+    }
+  },
+
+  deleteTodoItem: async (tripId, id) => {
+    const prev = get().todoItems
+    set(state => ({ todoItems: state.todoItems.filter(item => item.id !== id) }))
+    try {
+      await todoApi.delete(tripId, id)
+    } catch (err: unknown) {
+      set({ todoItems: prev })
+      throw new Error(getApiErrorMessage(err, 'Error deleting todo'))
+    }
+  },
+
+  toggleTodoItem: async (tripId, id, checked) => {
+    set(state => ({
+      todoItems: state.todoItems.map(item =>
+        item.id === id ? { ...item, checked: checked ? 1 : 0 } : item
+      )
+    }))
+    try {
+      await todoApi.update(tripId, id, { checked })
+    } catch {
+      set(state => ({
+        todoItems: state.todoItems.map(item =>
+          item.id === id ? { ...item, checked: checked ? 0 : 1 } : item
+        )
+      }))
+    }
+  },
+})