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_web/plugs/load_theme.ex

@@ -0,0 +1,39 @@
+defmodule SimpleshopThemeWeb.Plugs.LoadTheme do
+  @moduledoc """
+  Plug that loads theme settings and generated CSS for public shop pages.
+
+  This plug:
+  1. Checks the ETS cache for pre-generated CSS
+  2. Falls back to generating CSS from theme settings on cache miss
+  3. Assigns both `theme_settings` and `generated_css` to the connection
+
+  The generated CSS contains only the active theme values (not all variants),
+  making it much smaller than the full theme-layer2-attributes.css file used
+  by the theme editor for live preview switching.
+  """
+
+  import Plug.Conn
+
+  alias SimpleshopTheme.Settings
+  alias SimpleshopTheme.Theme.{CSSGenerator, CSSCache}
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    {theme_settings, generated_css} =
+      case CSSCache.get() do
+        {:ok, css} ->
+          {Settings.get_theme_settings(), css}
+
+        :miss ->
+          settings = Settings.get_theme_settings()
+          css = CSSGenerator.generate(settings)
+          CSSCache.put(css)
+          {settings, css}
+      end
+
+    conn
+    |> assign(:theme_settings, theme_settings)
+    |> assign(:generated_css, generated_css)
+  end
+end