fix(security): add JWT auth + visibility check to pages API

- GET /pages/:slug now requires JWT + checks custom_menu_items visibility
- GET /pages (list) is admin-only
- GET /pages/:slug/images/* uses visibility check without JWT (browser
  img tags cannot carry auth headers), blocks admin-only page images
- Frontend fetch adds Authorization header from authStore.token
- settingService nil guard changed to fail-closed (deny access)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/setting_service.go

@@ -1534,6 +1534,15 @@ func (s *SettingService) IsInvitationCodeEnabled(ctx context.Context) bool {
 	return value == "true"
 }
 
+// GetCustomMenuItemsRaw returns the raw JSON string of custom_menu_items setting.
+func (s *SettingService) GetCustomMenuItemsRaw(ctx context.Context) string {
+	value, err := s.settingRepo.GetValue(ctx, SettingKeyCustomMenuItems)
+	if err != nil {
+		return "[]"
+	}
+	return value
+}
+
 // IsAffiliateEnabled 检查是否启用邀请返利功能（总开关）
 func (s *SettingService) IsAffiliateEnabled(ctx context.Context) bool {
 	value, err := s.settingRepo.GetValue(ctx, SettingKeyAffiliateEnabled)