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/ui-design.md

@@ -113,6 +113,12 @@ MCP 面板
 - MCP 原有四个内层页签继续保留，外层和内层使用不同的 `aria-label` 与 `idPrefix`。
 - 窄屏时外层页签保持单行并允许水平滚动，不折成多行，不替换为下拉框。
 
+“MCP / 自定义 MCP”的添加和编辑共用设置上方的子 Modal。打开后聚焦名称输入框，
+父设置与工作区保持不可交互；Tab 和 Shift+Tab 在编辑器内循环。X、取消、Escape、
+遮罩及保存成功均返回原添加或编辑按钮，Escape 只关闭编辑器。保存期间禁止关闭，
+失败保留输入。右侧浏览器沿用应用级 Modal 的视图释放规则，编辑器关闭后仍由父设置
+保持隔离，设置关闭后恢复网页。
+
 ## 4. 工具下载源
 
 使用原生 Radio Group：