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

server/src/app.ts

@@ -18,6 +18,7 @@ import daysRoutes, { accommodationsRouter as accommodationsRoutes } from './rout
 import placesRoutes from './routes/places';
 import assignmentsRoutes from './routes/assignments';
 import packingRoutes from './routes/packing';
+import todoRoutes from './routes/todo';
 import tagsRoutes from './routes/tags';
 import categoriesRoutes from './routes/categories';
 import adminRoutes from './routes/admin';
@@ -179,6 +180,7 @@ export function createApp(): express.Application {
   app.use('/api/trips/:tripId/accommodations', accommodationsRoutes);
   app.use('/api/trips/:tripId/places', placesRoutes);
   app.use('/api/trips/:tripId/packing', packingRoutes);
+  app.use('/api/trips/:tripId/todo', todoRoutes);
   app.use('/api/trips/:tripId/files', filesRoutes);
   app.use('/api/trips/:tripId/budget', budgetRoutes);
   app.use('/api/trips/:tripId/collab', collabRoutes);