fix(flutter): non-blocking version check + 1h staleness throttle

Cold-start spinner was up to ~38s on slow / remote connections
because VersionGate blocked the entire ShellRoute on /healthz, with
the default dio's 8s connect + 30s receive timeouts. The /healthz
server handler itself is fine (microsecond JSON encode); the blocker
was client-side. Three issues fixed in one pass:

1. Optimistic render. VersionGate becomes a ConsumerStatefulWidget
   that always renders its child and just activates the version
   check controller on mount. The "you're too old" experience moves
   from a full-screen hard-block (_TooOldScreen, deleted) to a soft
   banner above the AppBar that lets the user keep playing cached
   content while they update.

2. 1h-throttled background check. New VersionCheckController
   (AsyncNotifier) hydrates from a secure-storage cache on boot,
   returning the cached result instantly. If the cache is missing
   or >1h old, fires a background recheck. AppLifecycleState.resumed
   triggers recheckIfStale so foregrounding after >1h re-checks
   without per-frame hammering. "Check now" button on the banner
   bypasses the staleness gate so dev iteration (push new APK, want
   to see banner clear) doesn't wait an hour.

3. Bounded health-check dio. The /healthz request uses a dedicated
   dio with connectTimeout: 3s + receiveTimeout: 2s rather than the
   default 8s / 30s. Health probes should fail fast — if the server
   can't ack in 5s, the user has bigger problems than a stale
   min_client_version and the cached value remains in effect.

Cache keys live alongside the existing tz cadence cache in
flutter_secure_storage (kResult + kAtMs). On any network error or
parse failure, _runCheck soft-fails without bumping the timestamp,
so the next staleness check will retry.

VersionTooOldBanner renders in _ShellWithPlayerBar's Column above
the existing UpdateBanner — the two coexist when both apply
(server rejects you AND an APK is queued).

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

flutter_client/lib/shared/routing.dart

@@ -151,6 +151,11 @@ class _ShellWithPlayerBar extends ConsumerWidget {
     final hasBanner = ref.watch(shouldShowUpdateBannerProvider) != null;
     return Column(
       children: [
+        // VersionTooOldBanner above UpdateBanner: a server-rejects-you
+        // soft warning carries more user-relevant urgency than the APK
+        // download prompt below it. The two banners can coexist
+        // (server says you're too old AND a local APK is queued).
+        const VersionTooOldBanner(),
         const UpdateBanner(),
         Expanded(
           child: hasBanner