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/screens/calendar/event_form_sheet.dart

@@ -120,12 +120,12 @@ class _EventFormSheetState extends ConsumerState<EventFormSheet> {
         'color': _color,
         'recurrence': rrule,
       };
-      final api = ref.read(eventsApiProvider);
+      final repo = ref.read(eventsRepositoryProvider);
       if (_isCreate) {
-        final created = await api.createEvent(payload);
+        final created = await repo.createEvent(payload);
         widget.notifier.addEvent(created);
       } else {
-        final updated = await api.updateEvent(widget.event!.id, payload);
+        final updated = await repo.updateEvent(widget.event!.id, payload);
         widget.notifier.updateEvent(updated);
       }
       if (mounted) Navigator.of(context).pop();
@@ -165,7 +165,7 @@ class _EventFormSheetState extends ConsumerState<EventFormSheet> {
     if (confirmed != true || !mounted) return;
     setState(() => _saving = true);
     try {
-      await ref.read(eventsApiProvider).deleteEvent(widget.event!.id);
+      await ref.read(eventsRepositoryProvider).deleteEvent(widget.event!.id);
       widget.notifier.removeEvent(widget.event!.id, widget.event!.startDt);
       if (mounted) Navigator.of(context).pop();
     } catch (_) {