Add go_router + Riverpod wiring and extract SOLID-clean screen/nav layers (na)

Introduces flutter_riverpod and go_router dependencies. Wires up the app
with a Riverpod-managed GoRouter, auth-guarded redirect logic, and a shared
navigator key used by DioClient for 401 → /login redirects.

SOLID fixes applied:
- navigatorKey extracted to navigation_key.dart (DIP: breaks cross-layer
  import from router.dart into api_client_provider.dart)
- AppRoutes extracted to app_routes.dart (SRP + avoids circular import
  when screen files reference route constants)
- LoginScreen, HomeScreen, MediaDetailScreen, ShareScreen each moved to
  their own file under lib/screens/ (SRP: router.dart is routing-only)
- router.dart re-exports AppRoutes for backward-compatible callers

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

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