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/model-connections/ui-design.md

@@ -22,6 +22,9 @@
   `aria-invalid` 并关联就地错误，避免只显示无法定位的“Invalid URL”。
 - LLM 模型连接的分类标题和模型类型分段页签在内容滚动时共同粘附在内容区顶部；左侧连接
   列表仍可独立滚动。
+- 连接列表和编辑表单按自身容器宽度切换左右或上下排列，断点及列表滚动上限遵循
+  [设置中心布局规则](../../../UI-DESIGN.md#138-设置中心)。窄窗口中收起的接口协议选择框
+  保留完整中英文当前值，不能依赖展开选项才能辨认协议。
 - 每个已保存或正在编辑的 LLM 连接提供“复制连接”。复制会生成新的连接名称和 ID，保留
   地址、模型、协议、请求定制、认证方式及当前草稿 API Key。已保存的 API Key 通过 Main
   进程的加密凭据复制路径保存，不回传到 Renderer；原连接和默认连接不改变。