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>

packages/node-runtime/src/services/index.ts

@@ -31,7 +31,9 @@ export {
   getFtsStatus,
   searchFts,
   rebuildFts,
+  getAllIndexStats,
 } from './session-index-service'
+export type { SessionIndexStatusItem } from './session-index-service'
 
 // Summary service
 export { generateSummary, generateAllSummaries } from './summary-service'

packages/node-runtime/src/services/session-index-service.ts

@@ -8,6 +8,7 @@ import {
   generateSessionIndex as coreGenerateSessionIndex,
   generateIncrementalSessionIndex as coreGenerateIncrementalSessionIndex,
   clearSessionIndex as coreClearSessionIndex,
+  getSessionIndexStats,
 } from '@openchatlab/core'
 import { hasFtsTable, searchByFts, rebuildFtsIndex } from '../fts'
 import type { SessionRuntimeAdapter } from './adapters'
@@ -51,3 +52,22 @@ export function rebuildFts(adapter: SessionRuntimeAdapter, sessionId: string) {
   const db = adapter.ensureWritable(sessionId)
   return rebuildFtsIndex(db)
 }
+
+export interface SessionIndexStatusItem {
+  sessionId: string
+  hasIndex: boolean
+  sessionCount: number
+}
+
+export function getAllIndexStats(adapter: SessionRuntimeAdapter): SessionIndexStatusItem[] {
+  const ids = adapter.listSessionIds()
+  return ids.map((sessionId) => {
+    try {
+      const db = adapter.ensureReadonly(sessionId)
+      const stats = getSessionIndexStats(db)
+      return { sessionId, hasIndex: stats.hasIndex, sessionCount: stats.sessionCount }
+    } catch {
+      return { sessionId, hasIndex: false, sessionCount: 0 }
+    }
+  })
+}