feat(offline): tier 2 phase 3 — write queue with optimistic UI

Offline writes now queue + apply optimistically instead of throwing.
On reconnect, the queue drains automatically; conflicts and rejections
surface as one-time SnackBars so the user knows their edit didn't land.

- Drift schema v3: `pending_writes` table (verb + payload + baseline +
  tries + last_error). Negative ids serve as cache placeholders for
  offline-created rows until replay assigns the server id.
- Each write repository (notes, tasks, projects, milestones, events)
  now catches NetworkException, applies the change to cache (with a
  fresh `nextTempId()` for creates), and enqueues the API call.
- Edits to a still-queued offline-created row coalesce into the
  original create payload — only one server call per row, in order.
- Deletes of still-queued offline-created rows drop the queue entry
  and the cache row; no server call ever happens.
- New WriteQueue service drains the queue oldest-first.
  Server-wins on `updated_at` baseline check (notes/tasks/projects);
  last-writer-wins for milestones/events (no getOne available).
  4xx → drop + surface as `rejected`; conflict → drop + `overwritten`;
  404 on update → drop + `missing`; 5xx/network → keep + retry next
  online cycle.
- Replay fires on AuthStatus → authenticated transitions
  (cold-start, came-back-online, login).
- OfflineBanner shows "Retry (N)" with the pending-writes count.
- Distinct ServerException added so 4xx no longer masquerade as
  NetworkException — the queue can drop them instead of looping.

flutter analyze clean; 21 tests pass (3 new for QueueFailure messaging).
Phase 4 (read-only UI indicators on cached/temp rows) still ahead.

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

lib/providers/api_client_provider.dart

@@ -25,6 +25,7 @@ import '../data/repositories/milestones_repository.dart';
 import '../data/repositories/notes_repository.dart';
 import '../data/repositories/projects_repository.dart';
 import '../data/repositories/tasks_repository.dart';
+import '../data/repositories/write_queue.dart';
 import 'settings_provider.dart';
 
 final cookieJarProvider = Provider<PersistCookieJar>((ref) {
@@ -148,3 +149,27 @@ final eventsRepositoryProvider = Provider<EventsRepository>((ref) {
     ref.watch(fabledDatabaseProvider),
   );
 });
+
+/// Phase 3 — drains the offline write queue. Listen on
+/// `writeQueueFailuresProvider` to surface dropped ops to the user;
+/// `writeQueueDepthProvider` powers the OfflineBanner's "Retry (N)" affordance.
+final writeQueueProvider = Provider<WriteQueue>((ref) {
+  final queue = WriteQueue(
+    db: ref.watch(fabledDatabaseProvider),
+    notesApi: ref.watch(notesApiProvider),
+    tasksApi: ref.watch(tasksApiProvider),
+    projectsApi: ref.watch(projectsApiProvider),
+    milestonesApi: ref.watch(milestonesApiProvider),
+    eventsApi: ref.watch(eventsApiProvider),
+  );
+  ref.onDispose(queue.dispose);
+  return queue;
+});
+
+final writeQueueDepthProvider = StreamProvider<int>((ref) {
+  return ref.watch(fabledDatabaseProvider).watchQueueDepth();
+});
+
+final writeQueueFailuresProvider = StreamProvider<QueueFailure>((ref) {
+  return ref.watch(writeQueueProvider).failures;
+});