feat(extensions): unify extensions endpoint and refresh extensions page UX

- Rename /home/plugins route to /home/extensions and update all sidebar links.
- Add unified GET /api/v1/extensions returning plugins, MCP servers and skills,
  sorted by name; replace the three separate frontend fetches with this single call.
- Migrate the extensions page to shadcn primitives (Tabs/Card/Alert/Badge/Skeleton/
  Switch/Label) and clean up hardcoded color tokens on the extension card.
- Add a localStorage-persisted "Group by type" switch that, when enabled in the
  All Types tab, renders extensions grouped by type with a compact section header.
- Show a spinner while loading and rename the empty-state copy from
  "No plugins installed" to "No extensions installed".
- Rename the "格式 / Formats" filter label to "类型 / Types" across all 8 locales.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/i18n/locales/zh-Hans.ts

@@ -433,6 +433,9 @@ const zhHans = {
     getPluginListError: '获取插件列表失败:',
     pluginConfig: '插件配置',
     noPluginInstalled: '暂未安装任何插件',
+    noExtensionInstalled: '暂未安装任何扩展',
+    loadingExtensions: '正在加载扩展...',
+    groupByType: '按类型分组',
     pluginSort: '插件排序',
     pluginSortDescription:
       '插件顺序会影响同一事件内的处理顺序，请拖动插件卡片排序',
@@ -634,7 +637,7 @@ const zhHans = {
       noTags: '暂无标签',
     },
     filters: {
-      allFormats: '全部格式',
+      allFormats: '全部类型',
       more: '更多',
       advancedTitle: '高级筛选',
       advancedDescription: '按扩展类型筛选',