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/client/index.tsx

@@ -16,6 +16,7 @@ import { createBetterSidebarService } from './service.ts'
 import { registerBuiltins } from './builtins/index.ts'
 import { Sidebar } from './Sidebar.tsx'
 import { registerOpenPathInterception, registerTurnTailInterception } from './intercept.tsx'
+import { registerLinkInterception } from './link-intercept.ts'
 import { loadPrefs } from './prefs.ts'
 import { SideCardSection } from './SideCardSection.tsx'
 import { api } from './api.ts'
@@ -158,6 +159,30 @@ export function apply(ctx: Context): void {
       'dsh-better-sidebar: open-path interception',
     )
 
+    ctx.effect(
+      () => {
+        try {
+          // External http(s) links in the chat/GUI open the sidebar browser
+          // instead of a new window (gated on the browserInterceptLinks pref
+          // AND the browser tab's enable switch; Ctrl/Cmd+click bypasses).
+          return registerLinkInterception({
+            takeoverEnabled: () => sidebarStore.getPrefs().browserInterceptLinks !== false
+              && sidebarStore.getPrefs().tabsEnabled['browser'] !== false,
+            openInSidebar: (url) => {
+              let title: string | undefined
+              try { title = new URL(url).hostname } catch { /* keep the default title */ }
+              ctx.betterSidebar?.openTab({ type: 'browser', url, title })
+            },
+            selfOrigin: window.location.origin,
+          })
+        } catch (error) {
+          fail('interception', error)
+          return undefined
+        }
+      },
+      'dsh-better-sidebar: link interception',
+    )
+
     // The "Side card" settings section: appears in the DSH Settings shell
     // once the shell's declaration is on the ledger (slots.inject waits for
     // it); the section reads/writes the prefs through the plugin's own