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>

lib/simpleshop_theme/settings.ex

@@ -83,6 +83,13 @@ defmodule SimpleshopTheme.Settings do
       settings = Ecto.Changeset.apply_changes(changeset)
       json = Jason.encode!(settings)
       put_setting("theme_settings", json, "json")
+
+      # Invalidate and rewarm CSS cache
+      alias SimpleshopTheme.Theme.{CSSCache, CSSGenerator}
+      CSSCache.invalidate()
+      css = CSSGenerator.generate(settings)
+      CSSCache.put(css)
+
       {:ok, settings}
     else
       {:error, changeset}