refactor(ui): simplify model button and optimize session index loading

- StatusBar model button: show model name instead of provider, click opens
  settings directly (scrolls to default model section), remove dropdown
- SessionIndexSection: replace N serial pluginQuery calls with single batch
  API (GET /_web/sessions/index-stats), add IntersectionObserver lazy loading
- Full-stack implementation: CLI route, Electron IPC/Worker, frontend adapters

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/cli/src/http/routes/web/session-index.ts

@@ -2,6 +2,10 @@ import type { FastifyInstance } from 'fastify'
 import { sessionIndexService, type SessionRuntimeAdapter } from '@openchatlab/node-runtime'
 
 export function registerSessionIndexRoutes(server: FastifyInstance, adapter: SessionRuntimeAdapter): void {
+  server.get('/_web/sessions/index-stats', async () => {
+    return sessionIndexService.getAllIndexStats(adapter)
+  })
+
   server.post<{
     Params: { id: string }
     Body: { gapThreshold?: number }