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

backend/internal/service/settings_view.go

@@ -154,8 +154,9 @@ type PublicSettings struct {
 	OIDCOAuthProviderName string
 	Version               string
 
-	BalanceLowNotifyEnabled    bool
-	AccountQuotaNotifyEnabled  bool
+	BalanceLowNotifyEnabled          bool
+	AccountQuotaNotifyEnabled        bool
+	BalanceLowNotifyThreshold        float64
 }
 
 // StreamTimeoutSettings 流超时处理配置（仅控制超时后的处理方式，超时判定由网关配置控制）