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/repositories/events_repository.dart

@@ -3,10 +3,13 @@ import '../api/events_api.dart';
 import '../local/database.dart';
 import '../models/calendar_event.dart';
 
-/// Calendar events repository with read-through caching for Tier 2 offline
-/// support. Range-scoped: each `getEvents(from, to)` mirrors that window into
-/// the cache (events outside the window are left alone) so successive
-/// disjoint range fetches don't clobber each other.
+/// Calendar events repository with read-through caching (Phase 1+2) and
+/// offline-write queueing (Phase 3). Range-scoped reads: `getEvents(from, to)`
+/// mirrors that window into the cache (events outside the window are left
+/// alone) so successive disjoint range fetches don't clobber each other.
+///
+/// CalendarEvent has no `updatedAt` field, so write replay uses last-
+/// writer-wins on update/delete (no server-side baseline check).
 class EventsRepository {
   final EventsApi _api;
   final FabledDatabase _db;
@@ -26,20 +29,116 @@ class EventsRepository {
   }
 
   Future<CalendarEvent> createEvent(Map<String, dynamic> payload) async {
-    final event = await _api.createEvent(payload);
-    await _db.upsertEvent(event);
-    return event;
+    try {
+      final event = await _api.createEvent(payload);
+      await _db.upsertEvent(event);
+      return event;
+    } on NetworkException {
+      final tempId = await _db.nextTempId();
+      final optimistic = _eventFromPayload(tempId, payload);
+      await _db.upsertEvent(optimistic);
+      await _db.enqueuePending(
+        domain: kSyncDomainEvents,
+        verb: kWriteVerbCreate,
+        tempId: tempId,
+        payload: payload,
+      );
+      return optimistic;
+    }
   }
 
   Future<CalendarEvent> updateEvent(
       int id, Map<String, dynamic> fields) async {
-    final updated = await _api.updateEvent(id, fields);
-    await _db.upsertEvent(updated);
-    return updated;
+    try {
+      final updated = await _api.updateEvent(id, fields);
+      await _db.upsertEvent(updated);
+      return updated;
+    } on NetworkException {
+      final cached = (await _db.getEventsInRange(
+        DateTime.fromMicrosecondsSinceEpoch(0),
+        DateTime.now().add(const Duration(days: 365 * 100)),
+      ))
+          .where((e) => e.id == id)
+          .firstOrNull;
+      if (cached == null) rethrow;
+      final optimistic = _applyEventFields(cached, fields);
+      await _db.upsertEvent(optimistic);
+      if (id < 0) {
+        final queued = await _db.findQueuedCreate(kSyncDomainEvents, id);
+        if (queued != null) {
+          await _db.updatePendingPayload(queued.id, fields);
+          return optimistic;
+        }
+      }
+      await _db.enqueuePending(
+        domain: kSyncDomainEvents,
+        verb: kWriteVerbUpdate,
+        targetId: id,
+        payload: fields,
+      );
+      return optimistic;
+    }
   }
 
   Future<void> deleteEvent(int id) async {
-    await _api.deleteEvent(id);
-    await _db.deleteEvent(id);
+    try {
+      await _api.deleteEvent(id);
+      await _db.deleteEvent(id);
+    } on NetworkException {
+      if (id < 0) {
+        final queued = await _db.findQueuedCreate(kSyncDomainEvents, id);
+        if (queued != null) await _db.deletePending(queued.id);
+        await _db.deleteEvent(id);
+        return;
+      }
+      await _db.deleteEvent(id);
+      await _db.enqueuePending(
+        domain: kSyncDomainEvents,
+        verb: kWriteVerbDelete,
+        targetId: id,
+      );
+    }
   }
 }
+
+CalendarEvent _eventFromPayload(int id, Map<String, dynamic> p) {
+  return CalendarEvent(
+    id: id,
+    title: p['title'] as String? ?? '',
+    startDt: _parseIso(p['start_dt'])!,
+    endDt: _parseIso(p['end_dt']),
+    allDay: p['all_day'] as bool? ?? false,
+    description: p['description'] as String? ?? '',
+    location: p['location'] as String? ?? '',
+    color: p['color'] as String? ?? '',
+    recurrence: p['recurrence'] as String?,
+    projectId: p['project_id'] as int?,
+    reminderMinutes: p['reminder_minutes'] as int?,
+  );
+}
+
+CalendarEvent _applyEventFields(CalendarEvent e, Map<String, dynamic> f) {
+  return CalendarEvent(
+    id: e.id,
+    title: f['title'] as String? ?? e.title,
+    startDt: _parseIso(f['start_dt']) ?? e.startDt,
+    endDt: f.containsKey('end_dt') ? _parseIso(f['end_dt']) : e.endDt,
+    allDay: f['all_day'] as bool? ?? e.allDay,
+    description: f['description'] as String? ?? e.description,
+    location: f['location'] as String? ?? e.location,
+    color: f['color'] as String? ?? e.color,
+    recurrence:
+        f.containsKey('recurrence') ? f['recurrence'] as String? : e.recurrence,
+    projectId: f.containsKey('project_id')
+        ? f['project_id'] as int?
+        : e.projectId,
+    reminderMinutes: f.containsKey('reminder_minutes')
+        ? f['reminder_minutes'] as int?
+        : e.reminderMinutes,
+  );
+}
+
+DateTime? _parseIso(dynamic raw) {
+  if (raw is String && raw.isNotEmpty) return DateTime.tryParse(raw)?.toLocal();
+  return null;
+}