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/milestones_repository.dart

@@ -3,8 +3,9 @@ import '../api/milestones_api.dart';
 import '../local/database.dart';
 import '../models/milestone.dart';
 
-/// Milestones repository with read-through caching for Tier 2 offline support.
-/// Mirrors the pattern in NotesRepository — see that file for full notes.
+/// Milestones repository with read-through caching (Phase 1+2) and
+/// offline-write queueing (Phase 3). See `notes_repository.dart` for the
+/// full pattern.
 class MilestonesRepository {
   final MilestonesApi _api;
   final FabledDatabase _db;
@@ -40,14 +41,45 @@ class MilestonesRepository {
     String? description,
     int orderIndex = 0,
   }) async {
-    final milestone = await _api.create(
-      projectId,
-      title: title,
-      description: description,
-      orderIndex: orderIndex,
-    );
-    await _db.upsertMilestone(milestone);
-    return milestone;
+    try {
+      final milestone = await _api.create(
+        projectId,
+        title: title,
+        description: description,
+        orderIndex: orderIndex,
+      );
+      await _db.upsertMilestone(milestone);
+      return milestone;
+    } on NetworkException {
+      final tempId = await _db.nextTempId();
+      final now = DateTime.now();
+      final optimistic = Milestone(
+        id: tempId,
+        projectId: projectId,
+        title: title,
+        description: description,
+        status: 'active',
+        orderIndex: orderIndex,
+        total: 0,
+        completed: 0,
+        pct: 0.0,
+        createdAt: now,
+        updatedAt: now,
+      );
+      await _db.upsertMilestone(optimistic);
+      await _db.enqueuePending(
+        domain: kSyncDomainMilestones,
+        verb: kWriteVerbCreate,
+        tempId: tempId,
+        payload: {
+          'project_id': projectId,
+          'title': title,
+          'description': description,
+          'order_index': orderIndex,
+        },
+      );
+      return optimistic;
+    }
   }
 
   Future<Milestone> update(
@@ -55,13 +87,78 @@ class MilestonesRepository {
     int milestoneId,
     Map<String, dynamic> fields,
   ) async {
-    final updated = await _api.update(projectId, milestoneId, fields);
-    await _db.upsertMilestone(updated);
-    return updated;
+    try {
+      final updated = await _api.update(projectId, milestoneId, fields);
+      await _db.upsertMilestone(updated);
+      return updated;
+    } on NetworkException {
+      final cached = (await _db.getMilestonesForProject(projectId))
+          .where((m) => m.id == milestoneId)
+          .firstOrNull;
+      if (cached == null) rethrow;
+      final optimistic = _applyMilestoneFields(cached, fields);
+      await _db.upsertMilestone(optimistic);
+      if (milestoneId < 0) {
+        final queued =
+            await _db.findQueuedCreate(kSyncDomainMilestones, milestoneId);
+        if (queued != null) {
+          // Carry project_id through coalesced payload — replay needs it.
+          final merged = <String, dynamic>{
+            'project_id': projectId,
+            ...fields,
+          };
+          await _db.updatePendingPayload(queued.id, merged);
+          return optimistic;
+        }
+      }
+      await _db.enqueuePending(
+        domain: kSyncDomainMilestones,
+        verb: kWriteVerbUpdate,
+        targetId: milestoneId,
+        payload: <String, dynamic>{'project_id': projectId, ...fields},
+        baselineUpdatedAt: cached.updatedAt,
+      );
+      return optimistic;
+    }
   }
 
   Future<void> delete(int projectId, int milestoneId) async {
-    await _api.delete(projectId, milestoneId);
-    await _db.deleteMilestone(milestoneId);
+    try {
+      await _api.delete(projectId, milestoneId);
+      await _db.deleteMilestone(milestoneId);
+    } on NetworkException {
+      if (milestoneId < 0) {
+        final queued =
+            await _db.findQueuedCreate(kSyncDomainMilestones, milestoneId);
+        if (queued != null) await _db.deletePending(queued.id);
+        await _db.deleteMilestone(milestoneId);
+        return;
+      }
+      await _db.deleteMilestone(milestoneId);
+      await _db.enqueuePending(
+        domain: kSyncDomainMilestones,
+        verb: kWriteVerbDelete,
+        targetId: milestoneId,
+        payload: {'project_id': projectId},
+      );
+    }
   }
 }
+
+Milestone _applyMilestoneFields(Milestone m, Map<String, dynamic> f) {
+  return Milestone(
+    id: m.id,
+    projectId: m.projectId,
+    title: f['title'] as String? ?? m.title,
+    description: f.containsKey('description')
+        ? f['description'] as String?
+        : m.description,
+    status: f['status'] as String? ?? m.status,
+    orderIndex: f['order_index'] as int? ?? m.orderIndex,
+    total: m.total,
+    completed: m.completed,
+    pct: m.pct,
+    createdAt: m.createdAt,
+    updatedAt: m.updatedAt,
+  );
+}