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/devices.js

@@ -150,7 +150,7 @@ export async function removeDevice(deviceId) {
             window.loadDevices();
         } else {
             const error = await response.json();
-            showToast(t('device.error.remove_failed'), 'error');
+            showToast(error.detail || t('device.error.remove_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;