fix(mobile): ignore unrelated mutes when building push subscriptions (#8608)

## Summary
Muting a starter channel (#welcome-everyone or #general) could leave the
iOS app on a blank screen after launch.

Starter channels use UUIDv5 IDs. Push subscription setup checked every
synced mute against the lease's UUIDv4 channel grammar and threw
`FormatException: Push subscription channel ID is invalid.` from
`pushSubscriptionSyncProvider` during build, so the first frame never
rendered. Whether you saw a blank screen depended on whether channels
and mutes finished loading before the first frame.

Mutes only feed the lease as ignore filters for subscribed (DM)
channels, and mutes sync across every community and channel type. So the
builder now matches mutes against the subscribed channels before using
them, instead of validating every mute. Subscribed channel IDs are still
validated as before.

### Related issue
None found.

### Testing
- Captured the failure on a dogfood iPhone build from the device system
log, with a stack trace through `buildDesiredBuzzPushSubscriptions` and
the muted ID `fee88e92-da9f-5958-a910-48db3264ad38` (#welcome-everyone).
Unmuting that channel from desktop fixed the device immediately, which
confirms the cause.
- New regression test `ignores mutes outside the push channel grammar`
fails on `main` with the same exception and passes with this change.
- The pre-push `mobile-checks` hook (`just mobile-check && just
mobile-test`) passed on this head.
- Screenshots: not applicable. This changes no UI. It only stops a
startup failure.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: morgmart <98432065+morgmart@users.noreply.github.com>
Co-authored-by: Carl <c217fe6b9d958f41c3a5e030dccc7f626775a923089cb6491305eade75ea1f1b@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

mobile/lib/shared/push/push_subscription.dart

@@ -456,9 +456,12 @@ List<BuzzPushSubscription> buildDesiredBuzzPushSubscriptions({
   }
 
   final normalizedChannelIds = channelIds.map(_normalizeChannelID).toSet();
-  final normalizedMuted = mutedChannelIds.map(_normalizeChannelID).toSet();
-  final activeMuted =
-      normalizedMuted.intersection(normalizedChannelIds).toList()..sort();
+  // Mutes sync across every community and channel type, so only mutes of
+  // subscribed channels matter here; others need not fit the push grammar.
+  final activeMuted = {
+    for (final id in mutedChannelIds)
+      if (normalizedChannelIds.contains(id.toLowerCase())) id.toLowerCase(),
+  }.toList()..sort();
   final mutedIgnoreFilters = <BuzzPushFilter>[];
   for (final chunk in _chunks(activeMuted, buzzPushChannelChunkSize)) {
     mutedIgnoreFilters.add(
@@ -489,7 +492,7 @@ List<BuzzPushSubscription> buildDesiredBuzzPushSubscriptions({
     ),
   ];
 
-  final channels = normalizedChannelIds.difference(normalizedMuted).toList()
+  final channels = normalizedChannelIds.difference(activeMuted.toSet()).toList()
     ..sort();
   for (final chunk in _chunks(channels, buzzPushChannelChunkSize)) {
     subscriptions.add(

mobile/test/shared/push/push_subscription_test.dart

@@ -93,6 +93,21 @@ void main() {
     );
   });
 
+  test('ignores mutes outside the push channel grammar', () {
+    // Starter channels use UUIDv5 IDs; muting one must not break push setup.
+    const starterChannel = 'fee88e92-da9f-5958-a910-48db3264ad38';
+
+    final subscriptions = buildDesiredBuzzPushSubscriptions(
+      myPubkey: me,
+      channelIds: [channelA],
+      mutedChannelIds: [starterChannel, 'not-a-channel'],
+    );
+
+    expect(subscriptions, hasLength(2));
+    expect(subscriptions.first.ignore, hasLength(1));
+    expect(subscriptions.last.filter.hTags, [channelA]);
+  });
+
   test('chunks channel subscriptions to relay limits deterministically', () {
     final channels = [
       for (var i = 0; i < 51; i++)