refactor: add themed form components for consistent shop styling

Adds reusable Phoenix components (shop_input, shop_textarea, shop_select,
shop_button, shop_card) backed by semantic CSS classes (.themed-input,
.themed-button, etc.) to eliminate repeated inline styles across templates.

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

lib/simpleshop_theme_web/components/page_templates/contact.html.heex

@@ -17,7 +17,7 @@
     <div class="grid gap-8 md:grid-cols-2 mb-12">
       <.contact_form email="hello@example.com" />
 
-      <div class="space-y-6">
+      <div class="flex flex-col gap-6">
         <.order_tracking_card />
 
         <.info_card title="Handy to know" items={[