feat(offline): #427 S1 — reachability offline marker

offlineProvider: a Notifier<bool> driven by a periodic /healthz
probe. Offline after N=3 consecutive failed probes; recovers on
the first success. Slow heartbeat when online (30s), faster when
offline (10s) so recovery is noticed quickly. 5s initial delay for
provider warmup; optimistic (false) until proven otherwise.

Deliberately NOT coupled to connectivityProvider — subscribing to
that StreamProvider eagerly mounts its 2s timeout and leaks a
pending Timer through widget tests (the MutationReplayer bug).
/healthz failing already covers interface-down. No .timeout()
wrapper either (dio's own timeouts bound the probe) so the only
Timers are the tracked initial+periodic, both cancelled via
ref.onDispose — the proven smoke-safe shape.

Wired in app.dart postFrame to start the poller. No UI yet; S4
gates system-playlist play + Shuffle-all on it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/app.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'cache/cache_filler.dart';
 import 'cache/metadata_prefetcher.dart';
+import 'cache/offline_provider.dart';
 import 'cache/mutation_queue.dart';
 import 'cache/prefetcher.dart';
 import 'cache/sync_controller.dart';
@@ -61,6 +62,10 @@ class _MinstrelAppState extends ConsumerState<MinstrelApp> {
       // scoring, scrobbles, and system-playlist rotation, and is the
       // path that carries the `source` tag for #415.
       ref.read(playEventsReporterProvider);
+      // Offline marker (#427 S1): periodic /healthz reachability
+      // probe → offlineProvider. Read here to start the poller; S4
+      // gates system-playlist play + Shuffle-all on it.
+      ref.read(offlineProvider);
     });
   }