feat(offline): tier 2 phase 2 — extend cache to tasks/projects/milestones/events/conversations

Phase 1 cached only notes. Phase 2 brings the read-through pattern to every
domain the app reads in bulk:

- Drift schema v2: 5 new cached_* tables + onUpgrade migration. Calendar
  events use range-scoped read/write (replaceEventsInRange) so disjoint
  month fetches don't clobber each other; milestones are per-project.
- Wrap TasksRepository, ProjectsRepository, MilestonesRepository, and
  ChatRepository (conversation list only) with NetworkException fallback.
  Writes hit the API then sync the cache.
- New EventsRepository wrapping EventsApi; calendar_provider and
  event_form_sheet repointed at the repository.
- OfflineBanner now surfaces getLatestSync() — most-recent timestamp
  across all domains — so the hint reads sensibly regardless of screen.

flutter analyze clean; existing tests pass. Phase 3 (offline write queue)
and Phase 4 (read-only UI indicators) still to come.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/data/repositories/notes_repository.dart

@@ -90,9 +90,4 @@ class NotesRepository {
     await _api.delete(id);
     await _db.deleteNote(id);
   }
-
-  /// Last successful `getAll` sync timestamp. Surfaced by the OfflineBanner
-  /// when the network is unreachable so the user knows how stale the
-  /// cached notes list is.
-  Future<DateTime?> lastSyncedAt() => _db.getLastSync(kSyncDomainNotes);
 }