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/setting_service.go

@@ -183,6 +183,7 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 		SettingKeyOIDCConnectEnabled,
 		SettingKeyOIDCConnectProviderName,
 		SettingKeyBalanceLowNotifyEnabled,
+		SettingKeyBalanceLowNotifyThreshold,
 		SettingKeyAccountQuotaNotifyEnabled,
 	}
 
@@ -222,6 +223,11 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 		settings[SettingKeyTablePageSizeOptions],
 	)
 
+	var balanceLowNotifyThreshold float64
+	if v, err := strconv.ParseFloat(settings[SettingKeyBalanceLowNotifyThreshold], 64); err == nil && v >= 0 {
+		balanceLowNotifyThreshold = v
+	}
+
 	return &PublicSettings{
 		RegistrationEnabled:              settings[SettingKeyRegistrationEnabled] == "true",
 		EmailVerifyEnabled:               emailVerifyEnabled,
@@ -253,6 +259,7 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 		OIDCOAuthProviderName:            oidcProviderName,
 		BalanceLowNotifyEnabled:          settings[SettingKeyBalanceLowNotifyEnabled] == "true",
 		AccountQuotaNotifyEnabled:        settings[SettingKeyAccountQuotaNotifyEnabled] == "true",
+		BalanceLowNotifyThreshold:        balanceLowNotifyThreshold,
 	}, nil
 }
 
@@ -308,6 +315,7 @@ func (s *SettingService) GetPublicSettingsForInjection(ctx context.Context) (any
 		Version                          string          `json:"version,omitempty"`
 		BalanceLowNotifyEnabled          bool            `json:"balance_low_notify_enabled"`
 		AccountQuotaNotifyEnabled        bool            `json:"account_quota_notify_enabled"`
+		BalanceLowNotifyThreshold        float64         `json:"balance_low_notify_threshold"`
 	}{
 		RegistrationEnabled:              settings.RegistrationEnabled,
 		EmailVerifyEnabled:               settings.EmailVerifyEnabled,
@@ -340,6 +348,7 @@ func (s *SettingService) GetPublicSettingsForInjection(ctx context.Context) (any
 		Version:                          s.version,
 		BalanceLowNotifyEnabled:          settings.BalanceLowNotifyEnabled,
 		AccountQuotaNotifyEnabled:        settings.AccountQuotaNotifyEnabled,
+		BalanceLowNotifyThreshold:        settings.BalanceLowNotifyThreshold,
 	}, nil
 }
 

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）