Add shipping weight support for Snipcart delivery calculation

- Add weight field to product blueprint (in grams)
- Add data-item-shippable and data-item-weight attributes to product template
- Enable Snipcart to calculate shipping costs based on product weight

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

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

site/templates/product.php

@@ -45,6 +45,8 @@
                         data-item-description="<?= $page->description()->excerpt(100) ?>"
                         data-item-image="<?= $page->images()->first() ? $page->images()->first()->url() : '' ?>"
                         data-item-name="<?= $page->title()->html() ?>"
+                        data-item-shippable="true"
+                        data-item-weight="<?= $page->weight()->or(0) ?>"
                         <?php
                         if($page->hasOptions()->toBool() && $page->optionValues()->isNotEmpty()):
                             $values = $page->optionValues()->split(',');