fix: delete confirm dialog never visible + race condition

- Add missing .confirm-dialog.active { display: flex; opacity: 1 } CSS rule.
  The confirm dialog was created with display:none and the .active class
  was added, but no CSS rule changed it to visible — so the user never
  saw the confirmation prompt and delete appeared to do nothing.
- Capture file/folder target before closeContextMenu in delete handlers
  to prevent null reference race condition (same as rename/share fix).

static/css/style.css

@@ -2489,6 +2489,11 @@ select:focus {
     animation: modalFadeIn 0.2s ease;
 }
 
+.confirm-dialog.active {
+    display: flex;
+    opacity: 1;
+}
+
 .confirm-dialog-content {
     background: white;
     border-radius: 16px;