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/prd.md

@@ -234,7 +234,7 @@ Runtime
 从当前聊天右栏迁移时，Task Center 和工作区作为默认打开、单实例且不可关闭的基础应用保留；
 浏览器保留一个默认打开实例，但注册为可关闭的多实例应用；成果保持默认打开、单实例且可
 关闭；终端不默认打开，由用户通过“+”创建，并从首期开始支持多实例。Tab 必须单行横向滚动，
-不能自动隐藏或缩写到不可辨认，“+”紧随最后一个 Tab 并随标签滚动，溢出时两端提供固定滚动箭头。Task Center 继续作为 Task 的现有
+不能自动隐藏或缩写到不可辨认。“+”固定在滚动区外的行末，溢出时页签区两端提供固定滚动箭头，箭头不覆盖“+”。Task Center 继续作为 Task 的现有
 入口；审批定位到所属 Task 或 Runtime。智能心跳不作为工作栏页签，其报告、建议、历史和
 完整配置统一归属“智能心跳”菜单入口。当前阶段不新增独立自动化中心。