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/infra/entities/api/index.ts

@@ -280,6 +280,15 @@ export interface ApiRespPlugins {
   plugins: Plugin[];
 }
 
+export type ExtensionItem =
+  | { type: 'plugin'; plugin: Plugin }
+  | { type: 'mcp'; server: MCPServer }
+  | { type: 'skill'; skill: Skill };
+
+export interface ApiRespExtensions {
+  extensions: ExtensionItem[];
+}
+
 export interface ApiRespPlugin {
   plugin: Plugin;
 }