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/chunks/docx.tsx

@@ -1,8 +0,0 @@
-/**
- * Lazy chunk entry: the .docx preview (docx-preview + jszip). Built as
- * `lib/client-docx.js` and registered under `dsh-better-sidebar/docx` —
- * fetched only when a .docx is first opened (see chunk-loader.ts and
- * docs/plans/2026-08-12-lazy-chunks-design.md). Never import this module
- * from the core bundle: it pulls the office library into the startup path.
- */
-export { DocxView } from '../docx-view.tsx'

src/client/chunks/pptx.tsx

@@ -1,8 +0,0 @@
-/**
- * Lazy chunk entry: the .pptx preview (pptx-renderer). Built as
- * `lib/client-pptx.js` and registered under `dsh-better-sidebar/pptx` —
- * fetched only when a .pptx is first opened (see chunk-loader.ts and
- * docs/plans/2026-08-12-lazy-chunks-design.md). Never import this module
- * from the core bundle: it pulls the renderer into the startup path.
- */
-export { PptxView } from '../PptxView.tsx'

src/client/chunks/xlsx.tsx

@@ -1,9 +0,0 @@
-/**
- * Lazy chunk entry: the .xlsx preview (Univer family + SheetJS, several MB).
- * Built as `lib/client-xlsx.js` and registered under
- * `dsh-better-sidebar/xlsx` — fetched only when a .xlsx is first opened
- * (see chunk-loader.ts and docs/plans/2026-08-12-lazy-chunks-design.md).
- * Never import this module from the core bundle: it pulls Univer into the
- * startup path.
- */
-export { XlsxView } from '../xlsx-view.tsx'