fix(mobile): stop oversized read-state retry loop (#4595)

## Summary

- stop retrying remote read-state publishes after the local replacement
blob exceeds NIP-44's 65,535-byte plaintext limit
- preserve every local read marker and leave existing relay state
untouched rather than truncating remote state
- keep incoming remote read-state available while suppressing further
invalid publishes for the manager lifetime

## Why

A repaired/reconnecting relay exposed a 1,404-context read-state on iOS.
The app repeatedly serialized and attempted to encrypt that structurally
oversized blob while reconnect catch-up work was running, saturating
Flutter's debug UI isolate and making channel navigation take roughly
ten seconds.

This is intentionally fail-closed and behavior-preserving: local read
behavior continues, but remote publishing pauses until the manager is
recreated. No protocol or persisted-data format changes.

## Verification

- `flutter test` — 1,093 passed, 1 skipped
- `flutter analyze` — no issues
- pre-push `mobile-test` and `branch-skew` hooks passed at
`0b6423c5d4d583194f0bbe69662912133b9ae1ef`
- independent review by Princess Donut: no blocking findings;
compatibility-safe and correctly fail-closed

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>

mobile/lib/shared/read_state/read_state_manager.dart

@@ -423,6 +423,16 @@ class ReadStateManager {
       _maxFetchedCreatedAt = max(_maxFetchedCreatedAt, createdAt);
       _persistLocalState();
     } catch (error) {
+      if (_isOversizedReadStateError(error)) {
+        _remoteUnsupported = true;
+        _debounceTimer?.cancel();
+        _debounceTimer = null;
+        debugPrint(
+          '[ReadStateManager] remote read-state sync disabled because the '
+          'local state exceeds the NIP-44 plaintext limit.',
+        );
+        return;
+      }
       if (_isPermanentReadStateRemoteError(error)) {
         _remoteUnsupported = true;
         _debounceTimer?.cancel();
@@ -526,6 +536,12 @@ class ReadStateManager {
   bool _isPlausibleCreatedAt(int createdAt) =>
       createdAt <= currentUnixSeconds() + readStateMaxClockDriftSeconds;
 
+  bool _isOversizedReadStateError(Object error) {
+    final msg = error.toString().toLowerCase();
+    return error is ArgumentError &&
+        msg.contains('plaintext must be 1-65535 bytes');
+  }
+
   bool _isPermanentReadStateRemoteError(Object error) {
     // Relay rejections come back as `Exception("<message>")` from the
     // websocket OK handler. Pattern-match on the message text since we no

mobile/test/features/channels/read_state/read_state_manager_test.dart

@@ -107,6 +107,41 @@ void main() {
     },
   );
 
+  test('disables remote sync after an oversized local blob', () async {
+    SharedPreferences.setMockInitialValues({});
+    final prefs = await SharedPreferences.getInstance();
+    final keychain = nostr.Keys.generate();
+    final crypto = ReadStateCrypto.tryCreate(
+      nsec: keychain.nsec,
+      pubkey: keychain.public,
+    )!;
+    final relay = _FakeSignedEventRelay();
+    final manager = ReadStateManager(
+      pubkey: keychain.public,
+      prefs: prefs,
+      crypto: crypto,
+      relaySession: null,
+      signedEventRelay: relay,
+      remoteEnabled: true,
+      onChanged: () {},
+    );
+
+    for (var index = 0; index < 1400; index++) {
+      manager.markContextRead(
+        'channel-${index.toString().padLeft(4, '0')}-${'x' * 48}',
+        index + 1,
+      );
+    }
+    await manager.flush();
+
+    manager.markContextRead('channel-new', 2000);
+    await manager.flush();
+
+    expect(relay.submitCount, 0);
+    expect(manager.getEffectiveTimestamp('channel-0000-${'x' * 48}'), 1);
+    expect(manager.getEffectiveTimestamp('channel-new'), 2000);
+  });
+
   test('remote read-state rollback is ignored', () async {
     SharedPreferences.setMockInitialValues({});
     final prefs = await SharedPreferences.getInstance();
@@ -171,6 +206,7 @@ NostrEvent _stubAckEvent() => const NostrEvent(
 
 class _FakeSignedEventRelay implements SignedEventRelay {
   final Completer<_SubmittedEvent> submitted = Completer<_SubmittedEvent>();
+  int submitCount = 0;
 
   @override
   String? get pubkey => null;
@@ -183,6 +219,7 @@ class _FakeSignedEventRelay implements SignedEventRelay {
     int? createdAt,
     void Function(NostrEvent event)? onSigned,
   }) async {
+    submitCount++;
     submitted.complete(_SubmittedEvent(kind: kind, tags: tags));
     return _stubAckEvent();
   }