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>

src/langbot/pkg/api/http/controller/groups/extensions.py

@@ -0,0 +1,52 @@
+from __future__ import annotations
+
+import asyncio
+import quart
+
+from .. import group
+
+
+@group.group_class('extensions', '/api/v1/extensions')
+class ExtensionsRouterGroup(group.RouterGroup):
+    """Unified API for installed extensions (plugins, MCP servers, skills)."""
+
+    async def initialize(self) -> None:
+        @self.route('', methods=['GET'], auth_type=group.AuthType.USER_TOKEN_OR_API_KEY)
+        async def _() -> quart.Response:
+            plugins, mcp_servers, skills = await asyncio.gather(
+                self.ap.plugin_connector.list_plugins(),
+                self.ap.mcp_service.get_mcp_servers(contain_runtime_info=True),
+                self.ap.skill_service.list_skills(),
+                return_exceptions=True,
+            )
+
+            def _sort_key(item: dict) -> str:
+                if item['type'] == 'plugin':
+                    return (
+                        item['plugin']
+                        .get('manifest', {})
+                        .get('manifest', {})
+                        .get('metadata', {})
+                        .get('name', '')
+                        .lower()
+                    )
+                if item['type'] == 'mcp':
+                    return (item['server'].get('name') or '').lower()
+                if item['type'] == 'skill':
+                    return (item['skill'].get('display_name') or item['skill'].get('name') or '').lower()
+                return ''
+
+            extensions: list[dict] = []
+            if isinstance(plugins, list):
+                for plugin in plugins:
+                    extensions.append({'type': 'plugin', 'plugin': plugin})
+            if isinstance(mcp_servers, list):
+                for server in mcp_servers:
+                    extensions.append({'type': 'mcp', 'server': server})
+            if isinstance(skills, list):
+                for skill in skills:
+                    extensions.append({'type': 'skill', 'skill': skill})
+
+            extensions.sort(key=_sort_key)
+
+            return self.success(data={'extensions': extensions})