feat: pin conversations and float their action menus

Persist conversation pins across restarts and show pinned conversations first, including older history. Anchor action menus above the list without shifting neighboring rows, with keyboard dismissal and sidebar-close cleanup.

Add schema 36 migration, focused regression coverage, and feature documentation. File-level staging also includes existing workbar styles and browser contract adjustments in shared files.

Validation: 145 backend tests and 237 App tests passed; typecheck and lint passed. Full test run timed out with Runtime and Agent package failures; native Electron visual validation remains pending.

Release note: 会话菜单新增置顶和取消置顶，重启后仍保留；操作菜单现在悬浮显示，不再挤动下方会话。

docs/features/assistant-workbar/README.md

@@ -10,7 +10,7 @@
 | [多终端页签 PRD](./terminal-tabs-prd.md) | 工作栏应用的固定、可关闭、单实例和多实例策略，以及多终端生命周期 |
 | [浏览器多实例技术设计](./browser-tabs-technical-design.md) | 浏览器 Tab 身份、Conversation 绑定、IPC、BrowserService 和 MCP 路由 |
 | [实现与验证进度](./progress.md) | 工作区与 Runtime 相关改动的实现事实、验证证据与剩余阻塞 |
-| [执行记录存储与升级回收](./execution-history-storage.md) | 子任务差量存储、远程去重、旧库转换、会话摘要与完整历史按需读取、内存引用释放 |
+| [执行记录存储与升级回收](./execution-history-storage.md) | 子任务差量存储、远程去重、旧库转换、会话置顶、摘要与完整历史按需读取、内存引用释放 |
 | [Runtime 交互边界](./runtime-interactions.md) | 原生权限、结构化问答、后台拒绝、终端交互、并行与输出、消息请求状态及资源生命周期规则 |
 | [Runtime 进程复用技术设计](./runtime-process-reuse-technical-design.md) | 跨 Runtime 资源调研、锁定协议证据、进程与会话拆分、远端模型桥、实施顺序和性能验收 |
 | [Runtime 执行清单技术设计](./runtime-checklist-technical-design.md) | OC/CN 原生清单提取、远程 Agent 传递复用、消息恢复与真实部署验收；尚未实现 |