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/live/shop_live/home.ex

@@ -0,0 +1,41 @@
+defmodule SimpleshopThemeWeb.ShopLive.Home do
+  use SimpleshopThemeWeb, :live_view
+
+  alias SimpleshopTheme.Settings
+  alias SimpleshopTheme.Media
+  alias SimpleshopTheme.Theme.{CSSCache, CSSGenerator, PreviewData}
+
+  @impl true
+  def mount(_params, _session, socket) do
+    # Load theme settings (cached CSS for performance)
+    theme_settings = Settings.get_theme_settings()
+
+    generated_css =
+      case CSSCache.get() do
+        {:ok, css} -> css
+        :miss ->
+          css = CSSGenerator.generate(theme_settings)
+          CSSCache.put(css)
+          css
+      end
+
+    logo_image = Media.get_logo()
+    header_image = Media.get_header()
+
+    preview_data = %{
+      products: PreviewData.products(),
+      categories: PreviewData.categories()
+    }
+
+    socket =
+      socket
+      |> assign(:page_title, "Home")
+      |> assign(:theme_settings, theme_settings)
+      |> assign(:generated_css, generated_css)
+      |> assign(:logo_image, logo_image)
+      |> assign(:header_image, header_image)
+      |> assign(:preview_data, preview_data)
+
+    {:ok, socket}
+  end
+end

lib/simpleshop_theme_web/live/shop_live/home.html.heex

@@ -0,0 +1,43 @@
+<div class="shop-container min-h-screen" style="background-color: var(--t-surface-base); font-family: var(--t-font-body); color: var(--t-text-primary);">
+  <.skip_link />
+
+  <%= if @theme_settings.announcement_bar do %>
+    <.announcement_bar theme_settings={@theme_settings} />
+  <% end %>
+
+  <.shop_header theme_settings={@theme_settings} logo_image={@logo_image} header_image={@header_image} active_page="home" mode={:shop} cart_count={0} />
+
+  <main id="main-content">
+    <.hero_section
+      title="Original designs, printed on demand"
+      description="From art prints to apparel – unique products created by independent artists and delivered straight to your door."
+      cta_text="Shop the collection"
+      cta_page="collection"
+      mode={:shop}
+    />
+
+    <.category_nav categories={@preview_data.categories} mode={:shop} />
+
+    <.featured_products_section
+      title="Featured products"
+      products={@preview_data.products}
+      theme_settings={@theme_settings}
+      mode={:shop}
+    />
+
+    <.image_text_section
+      title="Made with passion, printed with care"
+      description="Every design starts with an idea. We work with quality print partners to bring those ideas to life on premium products – from gallery-quality art prints to everyday essentials."
+      image_url="/mockups/mountain-sunrise-print-3.jpg"
+      link_text="Learn more about the studio →"
+      link_page="about"
+      mode={:shop}
+    />
+  </main>
+
+  <.shop_footer theme_settings={@theme_settings} mode={:shop} />
+
+  <.cart_drawer cart_items={[]} subtotal="£0.00" mode={:shop} />
+
+  <.search_modal hint_text={~s(Try searching for "mountain", "forest", or "ocean")} />
+</div>