fix(mall): prevent floor grid columns from overflowing container

`min-width: 0` on the flex item and `minmax(0, 1fr)` columns stop the
grid tracks from growing past their content and overflowing the
floor-grid's flex container.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/mall/pages/index.vue

@@ -194,8 +194,9 @@ const visibleFloors = computed(() => floors.value ?? []);
 }
 .floor-grid {
   flex: 1;
+  min-width: 0;
   display: grid;
-  grid-template-columns: repeat(4, 1fr);
+  grid-template-columns: repeat(4, minmax(0, 1fr));
   gap: 1px;
   background: var(--mall-line);
 }