feat(notify): add global toggles, percentage threshold, and visibility control

- Add global toggle for account quota notification in admin settings
- Add percentage-based threshold type for per-account quota alerts
- Hide balance notify card on user profile when global toggle is off
- Expose balance_low_notify_enabled and account_quota_notify_enabled in PublicSettings
- Add threshold type (fixed/percentage) to QuotaNotifyToggle with $ / % switcher

backend/internal/service/settings_view.go

@@ -112,7 +112,8 @@ type SystemSettings struct {
 	BalanceLowNotifyThreshold float64
 
 	// Account quota notification
-	AccountQuotaNotifyEmails []string
+	AccountQuotaNotifyEnabled bool
+	AccountQuotaNotifyEmails  []string
 }
 
 type DefaultSubscriptionSetting struct {
@@ -152,6 +153,9 @@ type PublicSettings struct {
 	OIDCOAuthEnabled      bool
 	OIDCOAuthProviderName string
 	Version               string
+
+	BalanceLowNotifyEnabled    bool
+	AccountQuotaNotifyEnabled  bool
 }
 
 // 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）