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/data/api/api_client.dart

@@ -63,5 +63,21 @@ AppException dioToApp(DioException e) {
   final status = e.response?.statusCode;
   if (status == 401) return const AuthException('Not authenticated.');
   if (status == 404) return const NotFoundException('Resource not found.');
+  if (status != null && status >= 400) {
+    final msg = _extractServerErrorMessage(e.response) ??
+        'Request failed (HTTP $status).';
+    return ServerException(msg, status);
+  }
   return NetworkException(e.message ?? 'Unknown network error.');
 }
+
+String? _extractServerErrorMessage(Response? resp) {
+  final data = resp?.data;
+  if (data is Map<String, dynamic>) {
+    for (final key in const ['detail', 'error', 'message']) {
+      final value = data[key];
+      if (value is String && value.isNotEmpty) return value;
+    }
+  }
+  return null;
+}