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/widgets/offline_banner.dart

@@ -43,8 +43,10 @@ class _OfflineBannerState extends ConsumerState<OfflineBanner> {
   Future<void> _loadLastSync() async {
     if (!mounted) return;
     try {
-      final ts =
-          await ref.read(notesRepositoryProvider).lastSyncedAt();
+      // Most-recent sync across all cached domains so the hint reads as
+      // "your data was current as of X" regardless of which screen the
+      // user is looking at.
+      final ts = await ref.read(fabledDatabaseProvider).getLatestSync();
       if (!mounted) return;
       setState(() => _lastSync = ts);
     } catch (_) {