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/live/theme_live/index.html.heex

@@ -252,7 +252,7 @@
             </div>
 
             <!-- Header Image Controls -->
-            <div class="mt-3 space-y-3">
+            <div class="mt-3 flex flex-col gap-3">
               <form phx-change="update_setting" phx-value-field="header_zoom">
                 <div class="flex justify-between items-center mb-2">
                   <span class="text-xs font-medium text-base-content/70">Zoom</span>