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/chat_repository.dart

@@ -1,21 +1,46 @@
+import '../../core/exceptions.dart';
 import '../api/chat_api.dart';
 export '../api/chat_api.dart'
     show ChatStreamEvent, ChatTextChunk, ChatStatusUpdate, ChatToolCall;
+import '../local/database.dart';
 import '../models/conversation.dart';
 import '../models/message.dart';
 
+/// Chat repository with read-through caching for the conversation list only.
+/// Messages and the live SSE stream are intentionally not cached — they are
+/// per-conversation, large, and require a live network anyway.
 class ChatRepository {
   final ChatApi _api;
-  const ChatRepository(this._api);
+  final FabledDatabase _db;
+
+  const ChatRepository(this._api, this._db);
+
+  Future<List<Conversation>> getConversations() async {
+    try {
+      final conversations = await _api.getConversations();
+      await _db.replaceAllConversations(conversations);
+      return conversations;
+    } on NetworkException {
+      final cached = await _db.getAllConversations();
+      if (cached.isEmpty) rethrow;
+      return cached;
+    }
+  }
 
-  Future<List<Conversation>> getConversations() => _api.getConversations();
   Future<Conversation> createConversation(String title) =>
       _api.createConversation(title);
-  Future<void> deleteConversation(int id) => _api.deleteConversation(id);
+
+  Future<void> deleteConversation(int id) async {
+    await _api.deleteConversation(id);
+    await _db.deleteConversation(id);
+  }
+
   Future<(Conversation, List<Message>)> getMessages(int conversationId) =>
       _api.getMessages(conversationId);
+
   Future<void> sendMessage(int conversationId, String content) =>
       _api.sendMessage(conversationId, content);
+
   Stream<ChatStreamEvent> streamGeneration(int conversationId) =>
       _api.streamGeneration(conversationId);
 }