Show backend error details in toast notifications

Use error.detail from API responses instead of generic i18n messages
so users see specific reasons for failures (e.g. "Device is referenced
by target(s): ...").

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/features/kc-targets.js

@@ -611,7 +611,7 @@ export async function deleteKCTarget(targetId) {
             if (typeof window.loadTargetsTab === 'function') window.loadTargetsTab();
         } else {
             const error = await response.json();
-            showToast(t('kc_target.error.delete_failed'), 'error');
+            showToast(error.detail || t('kc_target.error.delete_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;