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/app.dart

@@ -7,6 +7,7 @@ import 'package:flutter_timezone/flutter_timezone.dart';
 
 import 'core/constants.dart';
 import 'core/theme.dart';
+import 'data/repositories/write_queue.dart';
 import 'providers/api_client_provider.dart';
 import 'providers/auth_provider.dart';
 import 'core/exceptions.dart';
@@ -331,6 +332,17 @@ class _ShellState extends ConsumerState<_Shell> with WidgetsBindingObserver {
     );
   }
 
+  void _showQueueFailureSnackbar(QueueFailure failure) {
+    if (!mounted) return;
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(
+        content: Text(failure.message),
+        behavior: SnackBarBehavior.floating,
+        duration: const Duration(seconds: 6),
+      ),
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     // Show update dialog once when a new version is detected.
@@ -341,6 +353,23 @@ class _ShellState extends ConsumerState<_Shell> with WidgetsBindingObserver {
             .addPostFrameCallback((_) => _showUpdateSnackbar(next));
       }
     });
+
+    // Phase 3 — drain the offline write queue when we transition into
+    // an online state. Fires for unknown→authenticated (cold start),
+    // offline→authenticated (came back), and unauthenticated→authenticated
+    // (login). Idempotent if the queue is empty.
+    ref.listen(authProvider, (prev, next) {
+      if (next == AuthStatus.authenticated &&
+          prev != AuthStatus.authenticated) {
+        ref.read(writeQueueProvider).replay();
+      }
+    });
+
+    // Phase 3 — surface queue failures (overwrites, missing targets, 4xx
+    // rejections) so the user knows their offline edit didn't land.
+    ref.listen(writeQueueFailuresProvider, (_, next) {
+      next.whenData(_showQueueFailureSnackbar);
+    });
     final tabs = _tabs();
     final location = GoRouterState.of(context).matchedLocation;
     final index = _tabIndex(location, tabs);