feat(settings): 侧边栏设置声明式化 — 注册表驱动的开关清单 + 内置支持插头化

- SidebarPrefs 新增 tabsEnabled / viewersEnabled 开放 map（缺省 = 启用），
  PrefsSchema 用 z.dict 校验，旧文档零迁移
- TabDescriptor/FileViewerDescriptor 新增声明式设置面：viewer 补 title/icon，
  两者补 settings.toggles（子代理声明 autoOpenSubagent、终端声明 agentTerminalTools）
- 服务新增 isTabEnabled/isViewerEnabled；openTab 拒绝禁用类型、matchFileViewer
  跳过禁用 viewer
- SideCardSection 重写为注册表驱动清单：侧边栏内容（图标+类型id+开关+嵌套开关）
  + 文件预览（图标+标题+扩展名+开关），开关持久化 + 乐观提交回滚
- 消费点 gating：+ 菜单过滤、子代理自动展开、agent 终端 reconcile、
  产出文件拦截（editor 关闭时回落默认行）、新会话 explorer 预置
- 内置 viewer 补 6 个手绘 outline 图标；store 快照携带 prefs 使开关变更即时生效
- 测试：service gating / builtins 声明 / parsePrefs map 校验 / schema 断言 /
  新增 SideCardSection renderToString 规格
- 文档：README 设置小节、AGENTS.md 描述符与服务方法、新增设计文档；版本 0.5.0

src/client/icons.tsx

@@ -26,3 +26,60 @@ export const IconDiffOutline16 = ({ size = 16, className }: IconProps) => (
     <path d="M9.5 12.5h2.5" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" />
   </svg>
 )
+
+// ── File-viewer inventory glyphs (Side card settings page) ────────────────
+
+/** Image viewer glyph: a picture frame with a sun and a mountain. */
+export const IconImageOutline16 = ({ size = 16, className }: IconProps) => (
+  <svg width={size} height={size} className={className} viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <rect x="1.5" y="2.5" width="13" height="11" rx="2" stroke="currentColor" strokeWidth="1.5" />
+    <circle cx="5.5" cy="6" r="1.2" stroke="currentColor" strokeWidth="1.5" />
+    <path d="m3.5 12 3-3 2.25 2.25L11.5 8.5 13 10.5" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round" />
+  </svg>
+)
+
+/** PDF viewer glyph: a document frame with the "PDF" label. */
+export const IconPdfOutline16 = ({ size = 16, className }: IconProps) => (
+  <svg width={size} height={size} className={className} viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <path d="M3.5 1.5h6.5L13.5 5v9.5h-10z" stroke="currentColor" strokeWidth="1.5" strokeLinejoin="round" />
+    <path d="M9.5 1.5V5h4" stroke="currentColor" strokeWidth="1.5" strokeLinejoin="round" />
+    <path d="M5 13.5v-3h1.4c.75 0 1.1.32 1.1.85 0 .54-.35.85-1.1.85H5.3" stroke="currentColor" strokeWidth="1.25" strokeLinecap="round" strokeLinejoin="round" />
+    <path d="M8.3 13.5v-3h1.05c.8 0 1.35.5 1.35 1.5s-.55 1.5-1.35 1.5z" stroke="currentColor" strokeWidth="1.25" strokeLinecap="round" strokeLinejoin="round" />
+    <path d="M11.6 13.5v-3h1.3" stroke="currentColor" strokeWidth="1.25" strokeLinecap="round" />
+  </svg>
+)
+
+/** Word viewer glyph: a document frame with a "W". */
+export const IconDocxOutline16 = ({ size = 16, className }: IconProps) => (
+  <svg width={size} height={size} className={className} viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <path d="M3.5 1.5h6.5L13.5 5v9.5h-10z" stroke="currentColor" strokeWidth="1.5" strokeLinejoin="round" />
+    <path d="M9.5 1.5V5h4" stroke="currentColor" strokeWidth="1.5" strokeLinejoin="round" />
+    <path d="M6.2 13.4 7.4 10l1.2 3.4M7.4 10.6l-.35-1.1c-.2-.62.2-1.25.85-1.25h.2c.65 0 1.05.63.85 1.25l-.35 1.1" stroke="currentColor" strokeWidth="1.25" strokeLinecap="round" strokeLinejoin="round" />
+    <path d="M8.75 10.6 9.2 9.4" stroke="currentColor" strokeWidth="1.25" strokeLinecap="round" />
+  </svg>
+)
+
+/** Excel viewer glyph: a spreadsheet grid. */
+export const IconXlsxOutline16 = ({ size = 16, className }: IconProps) => (
+  <svg width={size} height={size} className={className} viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <rect x="1.5" y="2" width="13" height="12" rx="2" stroke="currentColor" strokeWidth="1.5" />
+    <path d="M1.5 6h13M1.5 9.5h13M6 6v8M10.5 6v8" stroke="currentColor" strokeWidth="1.25" />
+    <path d="m3.8 13.2 2-3M5.8 13.2l-2-3" stroke="currentColor" strokeWidth="1.25" strokeLinecap="round" />
+  </svg>
+)
+
+/** PowerPoint viewer glyph: a chart with rising bars. */
+export const IconPptxOutline16 = ({ size = 16, className }: IconProps) => (
+  <svg width={size} height={size} className={className} viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <rect x="1.5" y="2.5" width="13" height="11" rx="2" stroke="currentColor" strokeWidth="1.5" />
+    <path d="M4 12.5v-3M7 12.5V7M10 12.5V4.5M13 12.5v-1.5" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" />
+  </svg>
+)
+
+/** Markdown viewer glyph: the classic "M with a down arrow" badge. */
+export const IconMarkdownOutline16 = ({ size = 16, className }: IconProps) => (
+  <svg width={size} height={size} className={className} viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <rect x="1.5" y="2.5" width="13" height="11" rx="2" stroke="currentColor" strokeWidth="1.5" />
+    <path d="M4 10.5V5.5l2 2.5 2-2.5v5M9.5 10.5v-5l2 2.5 2-2.5v5" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round" />
+  </svg>
+)