feat(offline): detect server offline and keep returning users signed in

Previously, if the backend was unreachable at launch, the splash screen
routed to the login screen. The server URL remained persisted in
SharedPreferences but visually appeared lost, frustrating any user who
isn't the service operator.

- New AuthStatus.offline distinguishes network failures (NetworkException)
  from HTTP 401 in AuthNotifier.verify().
- Persist has_ever_logged_in flag on first successful verify/login.
- Offline + ever-logged-in lands on the briefing with a sticky offline
  banner (retry button) instead of being punted to login.
- OfflineBanner widget is Tier-2-ready so we can surface "last sync X min
  ago" once real caching lands (Fable task #147).

lib/screens/splash/splash_screen.dart

@@ -29,8 +29,13 @@ class _SplashScreenState extends ConsumerState<SplashScreen> {
     await ref.read(authProvider.notifier).verify();
     if (!mounted) return;
     final status = ref.read(authProvider);
+    final hasEverLoggedIn = ref.read(hasEverLoggedInProvider);
     if (status == AuthStatus.authenticated) {
       context.go(Routes.briefing);
+    } else if (status == AuthStatus.offline && hasEverLoggedIn) {
+      // Server unreachable but this user has logged in before — land them on
+      // the briefing with the offline banner rather than the login screen.
+      context.go(Routes.briefing);
     } else {
       context.go(Routes.login);
     }