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/ja-JP.ts

@@ -454,6 +454,9 @@ const jaJP = {
     loading: '読み込み中...',
     getPluginListError: 'プラグインリストの取得に失敗しました：',
     noPluginInstalled: 'プラグインがインストールされていません',
+    noExtensionInstalled: '拡張機能がインストールされていません',
+    loadingExtensions: '拡張機能を読み込み中...',
+    groupByType: '種類でグループ化',
     pluginConfig: 'プラグイン設定',
     pluginSort: 'プラグインの並び替え',
     pluginSortDescription:
@@ -659,7 +662,7 @@ const jaJP = {
       noTags: 'タグがありません',
     },
     filters: {
-      allFormats: 'すべての形式',
+      allFormats: 'すべての種類',
       more: 'もっと',
       advancedTitle: '高度なフィルター',
       advancedDescription: '拡張子タイプでフィルター',