feat: add mobile layout preview

apps/nextjs/src/components/board/mobile/mobile-preview.module.css

@@ -0,0 +1,39 @@
+.panel {
+  width: min(390px, calc(100vw - var(--mantine-spacing-xl)));
+  max-height: min(70vh, 700px);
+  overflow: hidden;
+}
+
+.viewport {
+  background: light-dark(var(--mantine-color-gray-1), var(--mantine-color-dark-8));
+}
+
+.grid {
+  display: grid;
+  grid-template-columns: repeat(var(--mobile-column-count), minmax(0, 1fr));
+  grid-auto-flow: row;
+  grid-auto-rows: 42px;
+  gap: var(--mantine-spacing-xs);
+}
+
+.item {
+  display: flex;
+  min-width: 0;
+  align-items: center;
+  justify-content: center;
+  border: 1px solid light-dark(var(--mantine-color-gray-4), var(--mantine-color-dark-4));
+  border-radius: var(--mantine-radius-sm);
+  background: light-dark(var(--mantine-color-white), var(--mantine-color-dark-6));
+}
+
+.itemContent {
+  min-width: 0;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .panel,
+  .item {
+    scroll-behavior: auto;
+    transition: none;
+  }
+}