feat(notify): improve balance notify card UX

- Show system default threshold as placeholder in custom threshold input
- Display user's primary email with "Primary" badge
- Support adding multiple pending emails before verification
- Each pending email has independent send/verify/resend flow
- Expose balance_low_notify_threshold in PublicSettings API
- Clean up timers on unmount to prevent leaks

frontend/src/types/index.ts

@@ -119,6 +119,7 @@ export interface PublicSettings {
   version: string
   balance_low_notify_enabled: boolean
   account_quota_notify_enabled: boolean
+  balance_low_notify_threshold: number
 }
 
 export interface AuthResponse {