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/local-tool-environment/technical-design.md

@@ -279,6 +279,14 @@ stdio MCP：
   Tab、不渲染占位面板。
 - 页签直接包装层使用 `flex: 0 0 auto`，并由 CSS 回归测试保证在滚动设置布局中不可收缩。
 
+`McpSettingsSection` 的添加与编辑共用 body Portal，遮罩使用 `--z-dialog`。
+打开前记录并聚焦触发按钮，再由 `activateModalFocus` 隔离父设置、聚焦名称并在关闭时
+恢复原隔离状态与焦点；键盘事件在子层停止传播。遮罩的 `mousedown` 关闭分支阻止默认
+焦点转移，避免恢复到触发按钮后又失焦。原生浏览器继续复用
+`browser-viewport-occlusion.ts`，无需 MCP 专用隐藏条件。
+真实 App 回归位于 `tests/overlay-layering.electron.test.ts`，使用生产 preload、IPC、
+配置存储及 `BrowserService`，验证添加、持久化、编辑、父层隔离与原生浏览器恢复。
+
 ### 11.2 工具环境状态与 IPC
 
 Renderer 接收：