Wire TabStateManager into GoRouter shell navigator

Use ref.listen inside the shell ConsumerStatefulWidget to react to GoRouter's currentConfiguration changes, rather than overriding didChangeDependencies — this avoids double-build issues. The onNavigationStateChange callback in StatefulShellRoute fires before the page animation completes, so updateStackSnapshot should capture the committed route list from the branch navigator's pages list, not from an in-progress transition. Guard against index-out-of-range if GoRouter emits a configuration before TabStateManager is fully hydrated by reading the current state synchronously before subscribing to the stream. Avoid wrapping the entire shell in a rebuild-heavy Consumer; instead, use select() to observe only the integer index so the shell itself does not rebuild when unrelated Riverpod state changes.

Unit tests (covered by task-011) verify TabStateManager logic in isolation. Integration tests for this wiring task should use flutter_test with a mock GoRouter and a fake TabStateManager to assert that every navigation event triggers the correct manager calls. Specifically: (1) tap tab 2 → setActiveTab(2) called once, (2) push sub-route on tab 2 → updateStackSnapshot called with correct stack, (3) pop back → updateStackSnapshot updated, (4) navigate to same tab → no duplicate setActiveTab call. Widget tests should pump the shell scaffold and verify the bottom nav selectedIndex matches the stream value without any setState.