Add Snipcart product validation endpoint and improve product gallery

- Add route handler for /validate.json to enable Snipcart product validation
- Update product gallery to display all product images instead of just the first one
- Fix CSS styling for product gallery list items
- Update home page to use first sorted image as product cover

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

site/templates/product.php

@@ -68,17 +68,23 @@
                 </div>
             </div>
 
-            <figure>
-                <?php if($image = $page->images()->first()): ?>
-                    <?php snippet('picture', [
-                        'file' => $image,
-                        'alt' => $page->title()->html(),
-                        'preset' => 'product-detail',
-                        'size' => 50,
-                        'lazy' => false
-                    ]) ?>
-                <?php endif ?>
-            </figure>
+            <?php if ($page->hasFiles()): ?>
+            <ul class="product-gallery">
+                <?php foreach($page->files()->sort() as $image): ?>
+                    <li>
+                        <figure>
+                            <?php snippet('picture', [
+                                'file' => $image,
+                                'alt' => $page->title()->html(),
+                                'preset' => 'product-detail',
+                                'size' => 50,
+                                'lazy' => false
+                            ]) ?>
+                        </figure>
+                    </li>
+                <?php endforeach ?>
+            </ul>
+            <?php endif ?>
         </section>
     </main>