perf: add defer to scripts and lazy loading to images

Add defer attribute to 145 <script> tags across 103 template files
(PERF-067) and loading="lazy" to 22 <img> tags across 13 template
files (PERF-058). Both improve page load performance.

Validator totals: 0 errors, 2 warnings, 1360 info (down from 1527).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/checkout/templates/checkout/storefront/checkout.html

@@ -384,7 +384,7 @@
                         <div class="divide-y divide-gray-200 dark:divide-gray-700">
                             <template x-for="item in cartItems" :key="item.product_id">
                                 <div class="py-4 flex items-center gap-4">
-                                    <img :src="item.image_url || '/static/shop/img/placeholder.svg'"
+                                    <img loading="lazy" :src="item.image_url || '/static/shop/img/placeholder.svg'"
                                          @error="$el.src = '/static/shop/img/placeholder.svg'"
                                          class="w-16 h-16 object-cover rounded-lg">
                                     <div class="flex-1 min-w-0">
@@ -428,7 +428,7 @@
                         <template x-for="item in cartItems" :key="item.product_id">
                             <div class="flex items-center gap-3">
                                 <div class="relative">
-                                    <img :src="item.image_url || '/static/shop/img/placeholder.svg'"
+                                    <img loading="lazy" :src="item.image_url || '/static/shop/img/placeholder.svg'"
                                          @error="$el.src = '/static/shop/img/placeholder.svg'"
                                          class="w-12 h-12 object-cover rounded">
                                     <span class="absolute -top-2 -right-2 w-5 h-5 bg-gray-500 text-white text-xs rounded-full flex items-center justify-center" x-text="item.quantity"></span>