extract shared theme CSS into standalone bundle

Move the 3 theme layer imports (primitives, layer2-attributes, semantic)
out of admin.css and shop.css into a new theme.css bundle loaded by all
root layouts. Eliminates 28 KB of duplication on admin pages where both
admin.css and shop.css were each embedding the same theme CSS.

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

config/config.exs

@@ -61,6 +61,10 @@ config :esbuild,
   berrypod_admin_css: [
     args: ~w(css/admin.css --bundle --outdir=../priv/static/assets/css),
     cd: Path.expand("../assets", __DIR__)
+  ],
+  berrypod_theme_css: [
+    args: ~w(css/theme.css --bundle --outdir=../priv/static/assets/css),
+    cd: Path.expand("../assets", __DIR__)
   ]
 
 # Configures Elixir's Logger