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/api/client.ts

@@ -136,6 +136,16 @@ export const packingApi = {
   deleteBag: (tripId: number | string, bagId: number) => apiClient.delete(`/trips/${tripId}/packing/bags/${bagId}`).then(r => r.data),
 }
 
+export const todoApi = {
+  list: (tripId: number | string) => apiClient.get(`/trips/${tripId}/todo`).then(r => r.data),
+  create: (tripId: number | string, data: Record<string, unknown>) => apiClient.post(`/trips/${tripId}/todo`, data).then(r => r.data),
+  update: (tripId: number | string, id: number, data: Record<string, unknown>) => apiClient.put(`/trips/${tripId}/todo/${id}`, data).then(r => r.data),
+  delete: (tripId: number | string, id: number) => apiClient.delete(`/trips/${tripId}/todo/${id}`).then(r => r.data),
+  reorder: (tripId: number | string, orderedIds: number[]) => apiClient.put(`/trips/${tripId}/todo/reorder`, { orderedIds }).then(r => r.data),
+  getCategoryAssignees: (tripId: number | string) => apiClient.get(`/trips/${tripId}/todo/category-assignees`).then(r => r.data),
+  setCategoryAssignees: (tripId: number | string, categoryName: string, userIds: number[]) => apiClient.put(`/trips/${tripId}/todo/category-assignees/${encodeURIComponent(categoryName)}`, { user_ids: userIds }).then(r => r.data),
+}
+
 export const tagsApi = {
   list: () => apiClient.get('/tags').then(r => r.data),
   create: (data: Record<string, unknown>) => apiClient.post('/tags', data).then(r => r.data),