wire shop.css alongside Tailwind for CSS migration (Phase 1)

- esbuild profile for shop.css bundling (dev watcher + build aliases)
- shop.css loaded as second stylesheet in shop_root layout
- LiveView display:contents rule in reset layer
- updated Lighthouse + Screenshots tasks for new esbuild target

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/shop.css

@@ -8,8 +8,9 @@
 
 @import "./shop/reset.css";
 
-/* Theme primitives and tokens stay in their existing files.
-   They'll be wrapped in @layer during Phase 1. */
+/* Theme CSS stays in the existing files (loaded via app-shop.css).
+   Primitives and tokens will be wrapped in @layer when
+   app-shop.css is removed in Phase 5. */
 
 @import "./shop/components.css";
 @import "./shop/layout.css";

assets/css/shop/reset.css

@@ -77,4 +77,10 @@
     padding: 0;
     cursor: pointer;
   }
+
+  /* LiveView wrapper divs are layout-invisible */
+  [data-phx-session],
+  [data-phx-teleported-src] {
+    display: contents;
+  }
 }