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/widgets/offline_banner.dart

@@ -84,9 +84,17 @@ class _OfflineBannerState extends ConsumerState<OfflineBanner> {
 
     final scheme = Theme.of(context).colorScheme;
     final ago = _lastSync == null ? null : _relativeAgo(_lastSync!);
-    final message = ago == null
+    // Pending queue depth — shown on the Retry button so the user knows
+    // there's offline work waiting to land when they come back online.
+    final pending = ref.watch(writeQueueDepthProvider).asData?.value ?? 0;
+
+    final baseMessage = ago == null
         ? 'Offline — showing cached data.'
         : 'Offline — last sync $ago.';
+    final message = pending > 0
+        ? '$baseMessage $pending pending change${pending == 1 ? '' : 's'}.'
+        : baseMessage;
+    final retryLabel = pending > 0 ? 'Retry ($pending)' : 'Retry';
 
     return Material(
       color: scheme.errorContainer,
@@ -117,7 +125,7 @@ class _OfflineBannerState extends ConsumerState<OfflineBanner> {
                         height: 14,
                         child: CircularProgressIndicator(strokeWidth: 2),
                       )
-                    : const Text('Retry'),
+                    : Text(retryLabel),
               ),
             ],
           ),