Implement GoRouter StatefulShellRoute configuration

Use `StatefulShellRoute.indexedStack` — not `StatefulShellRoute` with a custom navigator — to avoid state loss on tab switch. Each `StatefulShellBranch` should set `navigatorKey` to a branch-specific `GlobalKey` stored in `NavigationRouteConfig` to enable imperative navigation per branch. For `PopScope`, use `onPopInvokedWithResult` (Flutter 3.22+) to handle the `bool didPop` parameter correctly. The `canPop: false` on the root of each branch is the accessibility requirement from the workshops — users with motor impairments rely on explicit back buttons, not swipe gestures.

Implement a `redirect` callback on sensitive routes to check Riverpod auth state; use `ref.read` inside the redirect since the router exists outside the widget tree. Avoid nesting `GoRouter` instances — one router for the whole app. Register the router as `@riverpod GoRouter router(RouterRef ref)` so it can observe auth changes and trigger redirects.

Integration tests using `flutter_test` and `GoRouter`'s test utilities: (1) Confirm all five branches are navigable by name using `router.go(RouteConstants.homePath)` etc. (2) Confirm `PopScope` blocks swipe-back — simulate `Navigator.maybePop` and assert it returns `false`. (3) Confirm tab switching preserves sub-route: push a detail screen on Home tab, switch to Contacts, return to Home — detail screen must still be visible. (4) Confirm deep-link `/work/task/42` activates Work tab and pushes the task detail screen.

(5) Widget test: render the shell and verify five `BottomNavigationBar` items are present. Target: all five branches covered, swipe suppression verified on both platforms.