Use GoRouter redirects to handle app startup state changes

[bizz84]

Closes #150.

Motivation

While AppStartupWidget is a great way to initialize dependencies with eager provider initialization, particular care must be taken for apps that support navigation by URL or deep links.

For those apps, the root-level widget must be configured immediately (with GoRouter or equivalent).

To account for this, I've modified the GoRouter code as follows:

@riverpod
GoRouter goRouter(GoRouterRef ref) {
  // rebuild GoRouter when app startup state changes
  final appStartupState = ref.watch(appStartupProvider);
  final authRepository = ref.watch(authRepositoryProvider);
  return GoRouter(
    initialLocation: '/signIn',
    ...
    redirect: (context, state) {
      // If the app is still initializing, show the /startup route
      if (appStartupState.isLoading || appStartupState.hasError) {
        return '/startup';
      }
      ...
    },
    refreshListenable: GoRouterRefreshStream(authRepository.authStateChanges()),
    routes: [
      GoRoute(
        path: '/startup',
        pageBuilder: (context, state) => NoTransitionPage(
          child: AppStartupWidget(
            // * This is just a placeholder
            // * The loaded route will be managed by GoRouter on state change
            onLoaded: (_) => const SizedBox.shrink(),
          ),
        ),
      ),
      ...
    ],
    errorPageBuilder: (context, state) => const NoTransitionPage(
      child: NotFoundScreen(),
    ),
  );
}

Additionally, this PR moves the Firebase initialization code back in the main() function:

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  ...
  // * Initialize Firebase
  await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
  // * Entry point of the app
  runApp(const ProviderScope(
    child: MyApp(),
  ));
}

This prevents any "Firebase app not initialized" errors that would otherwise occur ref.watch(authRepositoryProvider) is called inside the GoRouter body.
Besides, the Firebase initialization code will only fail in case of a configuration error (which can be immediately spotted when running the app) - so it's best to keep this inside main().

[devloic]

I created a fresh clone of starter_architecture_flutter_firebase.
there is a redirect loop problem.

...
[GoRouter] Full paths for routes:
=> /startup
=> /onboard…edit
entries => /entries
profile => /account

[GoRouter] setting initial location /signIn
[GoRouter] redirecting to RouteMatchList(/onboarding)
[GoRouter] Redirection exception: redirect loop detected /signIn => /onboarding => /onboarding => /signIn
[GoRouter] Using MaterialApp configuration
[GoRouter] redirecting to RouteMatchList(/onboarding)
[GoRouter] Redirection exception: redirect loop detected /signIn => /onboarding => /onboarding => /signIn

I managed to bypass this problem so I could test the urls and they are working.

[bizz84]

I fixed the redirect loop here:
93d8aa6