feat: 设置页「添加插件」推荐目录 + 复制安装命令；移除内置 Office 预览

- 设置页侧边栏内容/文件预览两个网格末尾各增虚线卡片，弹出对应 kind 的插件弹窗
- 弹窗：GitHub topic 按钮（新标签页）+ 推荐插件目录（plugins-tabs.ts / plugins-viewers.ts）
- 每条目「跳转」（window.open 新标签页直达仓库）与「复制」（writeClipboard + 已复制反馈）
- 不做终端预填流：不关闭弹窗、不打开终端、无失败路径（install/installNotice/installError* 文案删除）
- 内置 Office 预览（docx/xlsx/pptx）整体移除，chunk 白名单 5→2（terminal/editor）
- 文案「内置插件」→「推荐插件」；AGENTS.md/README/设计文档同步

src/client/chunk-loader.ts

@@ -1,9 +1,11 @@
 /**
  * Lazy chunk loader for the client bundle. The heavy preview/terminal
- * libraries (Univer, docx-preview, pptx-renderer, CodeMirror, xterm — the
- * office/terminal/editor stacks, tens of MB) live in separate build-time
- * bundles (`lib/client-<name>.js`) fetched only on first use of the feature
- * that needs them, so startup downloads/parses only the ~1MB core bundle.
+ * libraries (CodeMirror, xterm — the editor/terminal stacks, several MB)
+ * live in separate build-time bundles (`lib/client-<name>.js`) fetched only
+ * on first use of the feature that needs them, so startup downloads/parses
+ * only the ~1MB core bundle. (The office stack — Univer / docx-preview /
+ * pptx-renderer — is no longer bundled here: Office previews moved to the
+ * recommended office plugin, see plugins-viewers.ts.)
  *
  * How a chunk script works (see tsdown.config.ts chunkBundle):
  *
@@ -42,7 +44,7 @@
  * open. Chunk-only source edits still need a manual page refresh (the HMR
  * poll watches only client.js).
  */
-export type ChunkName = 'docx' | 'xlsx' | 'pptx' | 'terminal' | 'editor'
+export type ChunkName = 'terminal' | 'editor'
 
 /** The module exports a chunk factory provides (namespace-ish record). */
 export type ChunkExports = Record<string, unknown>