feat: add shop storefront with optimized theme CSS

- Create LoadTheme plug for loading theme settings
- Add shop layout with inline CSS injection (~400 bytes vs 17KB full)
- Create ShopLive.Home at / using shared ShopComponents
- Wire up CSS cache invalidation when theme settings change
- Replace Phoenix welcome page with themed shop home page

The shop layout injects only the active theme CSS variables inline,
achieving a 97% reduction compared to the full variants file used
by the theme editor.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/simpleshop_theme_web/controllers/page_controller_test.exs

@@ -1,8 +1,8 @@
 defmodule SimpleshopThemeWeb.PageControllerTest do
   use SimpleshopThemeWeb.ConnCase
 
-  test "GET /", %{conn: conn} do
+  test "GET / renders the shop home page", %{conn: conn} do
     conn = get(conn, ~p"/")
-    assert html_response(conn, 200) =~ "Peace of mind from prototype to production"
+    assert html_response(conn, 200) =~ "Original designs, printed on demand"
   end
 end