feat: IMAP CONDSTORE fast-path, JMAP blob TTL, offline compose queue UI

- IMAP CONDSTORE (RFC 7162): skip sync when HIGHESTMODSEQ is unchanged;
  refresh only changed flags via CHANGEDSINCE on incremental sync
- JMAP blob expiry: re-fetch email bodies older than 7 days (schema v8→v9
  adds nullable cachedAt column to email_bodies)
- Offline compose queue: expose stuck pending_changes rows via
  observeFailedMutations / retryMutation / discardMutation; surface them
  in a FailedMutationBanner on the mailbox list screen
- Unit tests for all three features (236 passing)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/widget/helpers.dart

@@ -190,6 +190,16 @@ class FakeEmailRepository implements EmailRepository {
   @override
   Stream<void> watchJmapPush(String accountId, String password) =>
       const Stream.empty();
+
+  @override
+  Stream<List<FailedMutation>> observeFailedMutations(String accountId) =>
+      Stream.value([]);
+
+  @override
+  Future<void> discardMutation(int id) async {}
+
+  @override
+  Future<void> retryMutation(int id) async {}
 }
 
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