fix: restore consistent controls, usable overlays and narrow layouts

Resolve 15 shared-control, overlay and responsive-layout issues plus the MCP editor being hidden behind settings. Restore theme contrast, danger and disabled states, visible keyboard focus, and consistent Continue fields. Keep the workbar add button reachable and model protocol values readable in narrow containers.

Make nested MCP and image dialogs usable above their parents with focus restoration, keep project Escape handling within the active modal, release native browser views for overlapping help, and restore keyboard access and Escape in sandboxed HTML previews without relaxing sandbox permissions. Add unit and real Electron regression coverage and update design and validation documentation.

Release note: 修复深浅主题下控件边框、危险操作、禁用状态和键盘焦点显示不一致的问题。MCP 编辑器与图片预览可在父弹窗上正常操作，HTML 全屏预览支持键盘进入和 Escape 返回；窄工作栏的添加按钮保持可用，模型连接协议文字完整可读。

Validation: the full suite recorded 5309 passed, 3 failed and 83 skipped. The ProjectSwitcher initial-focus regression was then fixed, and App, ProjectSwitcher and overlay Electron tests passed all 331 cases. The other two HeartbeatCenter failures were reproduced on isolated HEAD 11facf0 and remain pre-existing. Typecheck and lint passed after the fix; staged whitespace checks passed. The full suite was not rerun after the focused fix.

Limits: Electron evidence is from Windows development runs, with fixtures for unrelated services and native occlusion throttling disabled in some drivers. No production build, packaging, macOS/Linux, remote Agent host or external MCP connection validation; skipped tests and the documented scaling and native-select coverage gaps remain.

docs/features/assistant-workbar/progress.md

@@ -1,5 +1,14 @@
 # 工作栏实现与验证进度
 
+## 2026-09-27：窄栏添加入口
+
+“+”已移出页签滚动容器；左右箭头和溢出测量使用扣除添加按钮宽度后的页签区域。
+Windows Electron 回归覆盖 `300px` 工作栏、默认四页签和 16 个含长标题的页签，验证
+原生鼠标打开应用目录、创建浏览器实例、Escape 返回、箭头滚动及 Home/End 激活。
+窄栏扩到 `900px` 后，默认页签的溢出箭头消失，缩回后恢复。
+中英文、浅深主题均通过，模型调用 0 次。范围与命令见
+[窄容器布局修复记录](../../quality/responsive-controls-review-2026-09-27.md)。
+
 ## 2026-09-27 Continue Update Status
 
 The shared Continue adapter now skips the update service before it emits the