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/plugins-shared.ts

@@ -0,0 +1,26 @@
+/**
+ * Shared vocabulary of the recommended plugin catalogs: the entry shape and
+ * the GitHub topic URL. The two catalogs live in sibling modules —
+ * `plugins-tabs.ts` (tab registrations) and `plugins-viewers.ts` (file
+ * previewer registrations) — and are shown in the two "add plugin" modals
+ * (Side card settings → the dashed cards at the end of the 侧边栏内容 /
+ * 文件预览 grids).
+ */
+
+/** The GitHub topic page listing every repo tagged `dsh-better-sidebar`. */
+export const PLUGIN_TOPIC_URL = 'https://github.com/topics/dsh-better-sidebar'
+
+/** One curated plugin entry (name / url / description / install script). */
+export interface PluginEntry {
+  /** Unique id (the npm package name). */
+  id: string
+  /** Short display name. */
+  name: string
+  /** GitHub repository URL. */
+  url: string
+  /** One-line description (i18n friendly: string or () => string). */
+  description: string | (() => string)
+  /** The full shell command pre-filled into the install terminal (not
+   *  executed until the user presses Enter). */
+  install: string
+}