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/core/exceptions.dart

@@ -17,3 +17,12 @@ class AuthException extends AppException {
 class NotFoundException extends AppException {
   const NotFoundException(super.message);
 }
+
+/// 4xx/5xx response received from the server (excluding 401/404 which have
+/// dedicated subclasses). Distinguished from `NetworkException` (connection
+/// failure) so the offline write queue can drop non-retryable failures
+/// instead of looping forever on a 422.
+class ServerException extends AppException {
+  final int statusCode;
+  const ServerException(super.message, this.statusCode);
+}