feat(chat): smooth loading with progressive session hydration

electron/preload.ts

@@ -130,6 +130,7 @@ contextBridge.exposeInMainWorld('electronAPI', {
   chat: {
     connect: () => ipcRenderer.invoke('chat:connect'),
     getSessions: () => ipcRenderer.invoke('chat:getSessions'),
+    getSessionStatuses: (usernames: string[]) => ipcRenderer.invoke('chat:getSessionStatuses', usernames),
     getExportTabCounts: () => ipcRenderer.invoke('chat:getExportTabCounts'),
     getSessionMessageCounts: (sessionIds: string[]) => ipcRenderer.invoke('chat:getSessionMessageCounts', sessionIds),
     enrichSessionsContactInfo: (usernames: string[]) =>