Add multi-user support with export feature

- New users table (migration 004) with user_id on exercises, training_sets, sessions
- User CRUD endpoints (GET/POST /api/v1/users, DELETE /api/v1/users/{id})
- All existing endpoints scoped to X-User-ID header
- CSV export endpoint (GET /api/v1/export) for completed sessions
- UserGate in PageShell: blocks app until a user is selected
- Settings page for managing users (create, switch, delete)
- BottomNav/Sidebar updated with settings navigation
- Fix: nil pointer panic in handleDeleteUser on success path
- Fix: export download now uses fetch with X-User-ID header instead of window.location.href

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -4,6 +4,7 @@ import { ExercisesPage } from './pages/ExercisesPage';
 import { SetsPage } from './pages/SetsPage';
 import { TrainingPage } from './pages/TrainingPage';
 import { HistoryPage } from './pages/HistoryPage';
+import { SettingsPage } from './pages/SettingsPage';
 
 const router = createBrowserRouter([
   {
@@ -13,6 +14,7 @@ const router = createBrowserRouter([
       { path: '/sets', element: <SetsPage /> },
       { path: '/training', element: <TrainingPage /> },
       { path: '/history', element: <HistoryPage /> },
+      { path: '/settings', element: <SettingsPage /> },
     ],
   },
 ]);