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/desktop/preload/apis/utils.ts

@@ -256,6 +256,13 @@ export const sessionApi = {
     return ipcRenderer.invoke('session:getStats', sessionId)
   },
 
+  /**
+   * 批量获取所有会话的索引状态
+   */
+  getAllIndexStats: (): Promise<Array<{ sessionId: string; hasIndex: boolean; sessionCount: number }>> => {
+    return ipcRenderer.invoke('session:getAllIndexStats')
+  },
+
   /**
    * 清空会话索引
    */

apps/desktop/preload/index.d.ts

@@ -1128,6 +1128,7 @@ interface SessionApi {
   generateIncremental: (sessionId: string, gapThreshold?: number) => Promise<number>
   hasIndex: (sessionId: string) => Promise<boolean>
   getStats: (sessionId: string) => Promise<SessionStats>
+  getAllIndexStats: () => Promise<Array<{ sessionId: string; hasIndex: boolean; sessionCount: number }>>
   clear: (sessionId: string) => Promise<boolean>
   updateGapThreshold: (sessionId: string, gapThreshold: number | null) => Promise<boolean>
   getSessions: (sessionId: string) => Promise<ChatSessionItem[]>