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/en-US.ts

@@ -449,6 +449,9 @@ const enUS = {
     loading: 'Loading...',
     getPluginListError: 'Failed to get plugin list:',
     noPluginInstalled: 'No plugins installed',
+    noExtensionInstalled: 'No extensions installed',
+    loadingExtensions: 'Loading extensions...',
+    groupByType: 'Group by type',
     pluginConfig: 'Plugin Configuration',
     pluginSort: 'Plugin Sort',
     pluginSortDescription:
@@ -658,7 +661,7 @@ const enUS = {
     deprecatedTooltip:
       'Please install the corresponding Knowledge Engine plugin.',
     filters: {
-      allFormats: 'All Formats',
+      allFormats: 'All Types',
       more: 'More',
       advancedTitle: 'Advanced Filters',
       advancedDescription: 'Filter by extension type',