Fix hit-testing: prefer child frame_bufs over parents

frame_buf_at was returning the parent frame instead of child windows
(titlebar, content area) because the parent was later in the list.
Now prefers children (frame_bufs with parent_id set) over parents.

This fixes menu titlebar click detection — clicking the titlebar now
dispatches to titlebarMouseDown/menuTitleMouseDown instead of
menuMouseDown, enabling menu pinning and drag-to-move.

src/backend/wayland/wl_backend.c

@@ -3555,8 +3555,11 @@ frame_buf_at(int px, int py)
         if (!fb->scene_buf || !fb->scene_buf->node.enabled)
             continue;
         if (px >= fb->x && px < fb->x + fb->width &&
-            py >= fb->y && py < fb->y + fb->height)
-            hit = fb;  /* last match = topmost in scene order */
+            py >= fb->y && py < fb->y + fb->height) {
+            /* Prefer children (more specific) over parents */
+            if (!hit || fb->parent_id)
+                hit = fb;
+        }
     }
     return hit;
 }