feat(tsk_PbcHccuPmwxV): 任务取消/删除 UI 入口（带确认）

## 后端
- Store.deleteTask()：BFS 收集子孙 id，清理 events，删根节点（子孙/runs/approvals 由 FK CASCADE 自动清除），广播 task.updated{kind:'delete'}
- API POST /api/tasks/:id/cancel：transition → cancelled（复用状态机守卫）
- API DELETE /api/tasks/:id：级联删除任务及子孙

## 前端
- 任务行（task-row）：鼠标悬停后右侧出现「✕取消」「🗑删除」浮动按钮，点击弹 confirm() 二次确认（含子孙数提示），stopPropagation 防止触发行展开
- 任务详情（detail）：底部行内确认区——「取消任务」显示确认条（确认取消 / 算了），「删除」显示含级联提示的高亮确认条，折叠/切换项目时自动清除确认状态
- 归档详情对话框：底部增加「🗑 删除归档」按钮，confirm() 确认后删除并关闭弹窗
- CSS：.task-row-cancel/.task-row-delete（悬停淡入）、.detail-actions / .detail-action-confirm / .detail-confirm-msg、.archive-modal-actions

## 测试
- 新增 4 个 store 测试：deleteTask 叶子、deleteTask 级联、deleteTask 不存在、transition→cancelled
- 全部 78 条 pass

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/store.test.ts

@@ -189,6 +189,56 @@ test('createTask：deps 引用不存在/跨项目任务被拒绝', () => {
   s.close();
 });
 
+test('deleteTask：删除叶子任务，不影响同项目其他任务', () => {
+  const s = freshStore();
+  const p = s.createProject({ name: 'del1', repoPath: '/tmp/del1-' + Math.random() });
+  const a = s.createTask({ projectId: p.id, title: 'A', complexity: 'easy' });
+  const b = s.createTask({ projectId: p.id, title: 'B', complexity: 'easy' });
+  const { deleted } = s.deleteTask(a.id);
+  assert.equal(deleted, 1);
+  assert.equal(s.getTask(a.id), null);       // 已删
+  assert.ok(s.getTask(b.id));               // 不受影响
+  s.close();
+});
+
+test('deleteTask：级联删除带子任务的父任务', () => {
+  const s = freshStore();
+  const p = s.createProject({ name: 'del2', repoPath: '/tmp/del2-' + Math.random() });
+  const root = s.createTask({ projectId: p.id, title: 'root', complexity: 'hard' });
+  s.setPlan(root.id, '拆'); s.transition(root.id, 'plan_review'); s.decide(root.id, 'accept', 'user');
+  const c1 = s.createTask({ projectId: p.id, parentId: root.id, title: 'c1', complexity: 'easy' });
+  const c2 = s.createTask({ projectId: p.id, parentId: root.id, title: 'c2', complexity: 'easy' });
+  // c1 下再建孙子
+  const gc = s.createTask({ projectId: p.id, parentId: c1.id, title: 'gc', complexity: 'easy' });
+  const { deleted } = s.deleteTask(root.id);
+  assert.equal(deleted, 4);                 // root + c1 + c2 + gc
+  assert.equal(s.getTask(root.id), null);
+  assert.equal(s.getTask(c1.id), null);
+  assert.equal(s.getTask(c2.id), null);
+  assert.equal(s.getTask(gc.id), null);
+  s.close();
+});
+
+test('deleteTask：不存在的任务抛 StoreError', () => {
+  const s = freshStore();
+  assert.throws(() => s.deleteTask('tsk_nonexistent'), (e: Error) => e.message.includes('任务不存在'));
+  s.close();
+});
+
+test('transition → cancelled：合法的状态流转（init/ready/executing 等）', () => {
+  const s = freshStore();
+  const p = s.createProject({ name: 'can', repoPath: '/tmp/can-' + Math.random() });
+
+  const t1 = s.createTask({ projectId: p.id, title: 'init task', complexity: 'easy' });
+  assert.equal(t1.status, 'ready');
+  const cancelled = s.transition(t1.id, 'cancelled', { by: 'user' });
+  assert.equal(cancelled.status, 'cancelled');
+
+  // 已取消的任务不可再次流转
+  assert.throws(() => s.transition(t1.id, 'ready'), (e: Error) => e.message.includes('非法状态流转'));
+  s.close();
+});
+
 test('容器收口：已拆解 Hard 的子任务全 done → 容器自动 done（逐级向上）', () => {
   const s = freshStore();
   const p = s.createProject({ name: 'cc', repoPath: '/tmp/cc-' + Math.random() });