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/stores/app.ts

@@ -342,6 +342,7 @@ export const useAppStore = defineStore('app', () => {
         version: siteVersion.value,
         balance_low_notify_enabled: false,
         account_quota_notify_enabled: false,
+        balance_low_notify_threshold: 0,
       }
     }