perf(export): cache counts and speed sns/session stats

src/types/electron.d.ts

@@ -554,6 +554,7 @@ export interface ElectronAPI {
     onExportProgress: (callback: (payload: { current: number; total: number; status: string }) => void) => () => void
     selectExportDir: () => Promise<{ canceled: boolean; filePath?: string }>
     getSnsUsernames: () => Promise<{ success: boolean; usernames?: string[]; error?: string }>
+    getExportStatsFast: () => Promise<{ success: boolean; data?: { totalPosts: number; totalFriends: number }; error?: string }>
     getExportStats: () => Promise<{ success: boolean; data?: { totalPosts: number; totalFriends: number }; error?: string }>
     installBlockDeleteTrigger: () => Promise<{ success: boolean; alreadyInstalled?: boolean; error?: string }>
     uninstallBlockDeleteTrigger: () => Promise<{ success: boolean; error?: string }>