feat: 内置 HTML 预览器 + 沙箱浏览器 tab（v0.5.0）

- html viewer：.html/.htm 沙箱 iframe 预览（相对资源经 /sidebar/html 路径编码路由加载，CSP sandbox + 信任围栏 + cwd 边界），预览/编辑切换
- browser tab：+ 菜单多开（browser:<n>），地址栏后退/前进/刷新/在浏览器中打开（15px 图标），URL 与标题按会话持久化（patchTab + nextBrowser）
- 安全：iframe 一律不透明源沙箱（无 allow-same-origin/top-navigation、no-referrer、权限策略全禁、禁 srcdoc）；地址栏仅 http(s)、拒绝 localhost 等本机地址；界面实时沙箱状态（关闭红色警示）+ 临时解锁/恢复
- 设置：htmlViewerNoSandbox / htmlViewerDefaultUnsafe / browserNoSandbox / browserInterceptLinks 四个声明式二级设置（默认安全）
- 外链拦截：聊天/界面 http(s) 外链默认在侧边栏打开（openTab seed.url），Ctrl/Cmd 点击放行
- 嵌入拒绝面板：browser.probe 路由探测 X-Frame-Options/frame-ancestors，被拒站点显示原因 + 在浏览器中打开/仍然加载
- 设置页可滚动；内置 7 tab + 9 viewer；新增设计文档；268 个测试全绿

src/prefs-shared.ts

@@ -35,6 +35,38 @@ export interface SidebarPrefs {
    * own enable switch gates it too (both must be on for the takeover).
    */
   interceptOpenPath: boolean
+  /**
+   * Whether the HTML previewer drops its sandboxed iframe. Sandbox ON (the
+   * default) renders previewed HTML in an opaque-origin iframe that cannot
+   * touch the GUI; turning it OFF runs the previewed page with the GUI's
+   * own origin — full read/write access to session files and internal
+   * APIs. Only for trusted local content; the setting copy warns.
+   */
+  htmlViewerNoSandbox: boolean
+  /**
+   * Whether a newly opened HTML preview starts UNSANDBOXED (the per-surface
+   * temporary unlock pre-applied). Off by default: previews open sandboxed
+   * and the status row offers the one-tap unlock; when on, previews open
+   * in the red unsandboxed state and the status row offers a one-tap
+   * restore for the current file.
+   */
+  htmlViewerDefaultUnsafe: boolean
+  /**
+   * Whether the browser tab drops its sandboxed iframe. Sandbox ON (the
+   * default) keeps browsed sites in an opaque origin with no GUI access;
+   * turning it OFF runs any visited site with the GUI's own origin — it
+   * can read session data and act as the logged-in GUI. Only for trusted
+   * sites; the setting copy warns.
+   */
+  browserNoSandbox: boolean
+  /**
+   * Whether clicking an http(s) EXTERNAL link in the GUI (chat messages,
+   * tool rows, prose mentions) opens the sidebar browser instead of a new
+   * browser tab. On by default; gated on the browser tab's own enable
+   * switch (both must be on for the takeover). Ctrl/Cmd+click always
+   * bypasses the takeover.
+   */
+  browserInterceptLinks: boolean
   /**
    * Per-tab enable switches, keyed by tab descriptor id (`'explorer'`,
    * `'my-plugin:db'`). An ABSENT key means enabled — only an explicit
@@ -66,6 +98,10 @@ export const SIDEBAR_PREFS_DEFAULTS: SidebarPrefs = {
   autoOpenSubagent: true,
   agentTerminalTools: false,
   interceptOpenPath: true,
+  htmlViewerNoSandbox: false,
+  htmlViewerDefaultUnsafe: false,
+  browserNoSandbox: false,
+  browserInterceptLinks: true,
   tabsEnabled: {},
   viewersEnabled: {},
 }