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/app/home/layout.tsx

@@ -45,7 +45,7 @@ import {
 
 // Routes that belong to the "Extensions" section
 const EXTENSIONS_ROUTES = [
-  '/home/plugins',
+  '/home/extensions',
   '/home/market',
   '/home/mcp',
   '/home/skills',
@@ -124,7 +124,7 @@ function HomeLayoutInner({ children }: { children: React.ReactNode }) {
   const sectionLabel = isExtensions
     ? t('sidebar.extensions')
     : t('sidebar.home');
-  const sectionLink = isExtensions ? '/home/plugins' : '/home/monitoring';
+  const sectionLink = isExtensions ? '/home/extensions' : '/home/monitoring';
 
   return (
     <SidebarProvider>