refactor(delete): make undo unconditional, remove undo toggle and button delay

- Remove delete_undo_enabled setting (backend default, frontend state,
  settings modal UI, 10 locales); staged deletes with 30s undo are now
  the only delete path and stale settings keys are silently ignored
- Remove the 1500ms delete-button arm delay (armDeleteButton) from all
  delete modals; misclicks are recoverable via the undo toast
- Delete modal always shows the recoverable warning
- Log the first staged file path in staging log lines for easier support

static/js/api/baseModelApi.js

@@ -203,7 +203,7 @@ export class BaseModelApiClient {
                 }
                 const batchId = data.batch_id || null;
                 if (!batchId) {
-                    // Not staged (undo disabled or staging failed): keep the legacy toast.
+                    // Not staged (staging failed): keep the legacy toast.
                     // When staged, the caller shows the undo action toast instead.
                     showToast('toast.api.deleteSuccess', { type: this.apiConfig.config.displayName }, 'success');
                 }