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>

config/config.exs

@@ -53,6 +53,10 @@ config :esbuild,
       ~w(js/app.js --bundle --target=es2022 --outdir=../priv/static/assets/js --external:/fonts/* --external:/images/* --alias:@=.),
     cd: Path.expand("../assets", __DIR__),
     env: %{"NODE_PATH" => [Path.expand("../deps", __DIR__), Mix.Project.build_path()]}
+  ],
+  simpleshop_theme_shop_css: [
+    args: ~w(css/shop.css --bundle --outdir=../priv/static/assets/css),
+    cd: Path.expand("../assets", __DIR__)
   ]
 
 # Configure tailwind (the version is required)