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/builtins/tabs.tsx

@@ -1,9 +1,10 @@
 /**
- * The 6 built-in tab descriptors: the plugin registers its own pages
- * (explorer / git / terminal / subagent / editor / diff) through the same
- * {@link BetterSidebarService} external plugins use — eating its own
- * dogfood. The terminal descriptor owns its quota (`TERMINAL_LIMIT`) and
- * mints `terminal:<n>` ids through `createTab`.
+ * The 7 built-in tab descriptors: the plugin registers its own pages
+ * (explorer / git / terminal / browser / subagent / editor / diff) through
+ * the same {@link BetterSidebarService} external plugins use — eating its
+ * own dogfood. The terminal descriptor owns its quota (`TERMINAL_LIMIT`)
+ * and mints `terminal:<n>` ids through `createTab`; the browser mints
+ * `browser:<n>` the same way (no quota).
  */
 import { IconBranchOutline16, IconCodeOutline16, IconFolderOpen16, IconThinkOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { Context } from '../../context-types.ts'
@@ -16,7 +17,8 @@ import { TerminalView } from '../TerminalView.tsx'
 import { GitView } from '../GitView.tsx'
 import { DiffTab } from '../DiffTab.tsx'
 import { SubagentView } from '../SubagentView.tsx'
-import { IconTerminalOutline16, IconDiffOutline16 } from '../icons.tsx'
+import { BrowserView } from '../BrowserView.tsx'
+import { IconTerminalOutline16, IconDiffOutline16, IconGlobeOutline16 } from '../icons.tsx'
 import type { TabDescriptor } from '../service.ts'
 
 /** How many UI-owned terminals may be open at once (agent-owned ones are uncapped). */
@@ -29,7 +31,7 @@ function uiTerminalCount(state: SidebarState): number {
     .filter(tab => tab.type === 'terminal' && !isAgentTabId(tab.id)).length
 }
 
-/** The 6 built-in tab descriptors. */
+/** The 7 built-in tab descriptors. */
 export function builtinTabs(ctx: Context): readonly TabDescriptor[] {
   return [
     {
@@ -130,6 +132,35 @@ export function builtinTabs(ctx: Context): readonly TabDescriptor[] {
         <TerminalView scope={scope} tabId={tab.id} store={store} />
       ),
     },
+    {
+      id: 'browser',
+      title: () => t('browser'),
+      icon: (size: number) => <IconGlobeOutline16 size={size} />,
+      order: 50,
+      // Declarative settings: the sandbox escape hatch and the link
+      // takeover render under this tab's row in the Side card settings
+      // page (the sandbox one is warned on).
+      settings: {
+        toggles: [{
+          key: 'browserNoSandbox',
+          title: () => t('settingsBrowserSandboxTitle'),
+          desc: () => t('settingsBrowserSandboxDesc'),
+        }, {
+          key: 'browserInterceptLinks',
+          title: () => t('settingsBrowserLinksTitle'),
+          desc: () => t('settingsBrowserLinksDesc'),
+        }],
+      },
+      createTab: (state) => ({
+        tab: {
+          id: `browser:${state.nextBrowser}`,
+          type: 'browser',
+          title: t('browser'),
+        },
+        patch: { nextBrowser: state.nextBrowser + 1 },
+      }),
+      component: (props) => <BrowserView {...props} />,
+    },
     {
       id: 'diff',
       title: () => t('git'),

src/client/builtins/viewers.tsx

@@ -1,8 +1,8 @@
 /**
- * The 8 built-in file viewer descriptors: every preview surface is a
- * registered viewer (image / pdf / docx / xlsx / pptx / markdown / code /
- * binary-download), exactly like external plugins register theirs. The
- * `binary-download` viewer sniffs NUL bytes via `detect` for unknown
+ * The 9 built-in file viewer descriptors: every preview surface is a
+ * registered viewer (image / pdf / docx / xlsx / pptx / markdown / html /
+ * code / binary-download), exactly like external plugins register theirs.
+ * The `binary-download` viewer sniffs NUL bytes via `detect` for unknown
  * binaries and serves doc/xls/ppt by extension; `code` is the catch-all
  * (`exts: []`, lowest priority) that claims any file no other viewer did.
  *
@@ -23,12 +23,13 @@ import {
   IconDocxOutline16,
   IconXlsxOutline16,
   IconPptxOutline16,
+  IconHtmlOutline16,
 } from '../icons.tsx'
 import type { FileViewerDescriptor } from '../service.ts'
 import { t } from '../locales.ts'
 import css from '../sidebar.module.css'
 
-/** The 8 built-in file viewer descriptors. */
+/** The 9 built-in file viewer descriptors. */
 export function builtinViewers(): readonly FileViewerDescriptor[] {
   return [
     {
@@ -91,6 +92,28 @@ export function builtinViewers(): readonly FileViewerDescriptor[] {
       fetchStrategy: 'fsRead',
       component: (props) => <TextEditor {...props} />,
     },
+    {
+      id: 'html',
+      title: () => t('viewerHtml'),
+      icon: (size: number) => <IconHtmlOutline16 size={size} />,
+      exts: ['html', 'htm'],
+      fetchStrategy: 'fsRead',
+      // Declarative settings: the sandbox escape hatch and the default-unsafe
+      // start state render under this viewer's row in the Side card settings
+      // page (both warned on).
+      settings: {
+        toggles: [{
+          key: 'htmlViewerNoSandbox',
+          title: () => t('settingsHtmlSandboxTitle'),
+          desc: () => t('settingsHtmlSandboxDesc'),
+        }, {
+          key: 'htmlViewerDefaultUnsafe',
+          title: () => t('settingsHtmlDefaultUnsafeTitle'),
+          desc: () => t('settingsHtmlDefaultUnsafeDesc'),
+        }],
+      },
+      component: (props) => <TextEditor {...props} />,
+    },
     {
       id: 'code',
       title: () => t('viewerCode'),