Add package dimensions for shipping calculation

- Add length, width, and height fields to product blueprint (in centimeters)
- Add corresponding data-item attributes for Snipcart shipping calculation
- Enable volume-based shipping cost calculation

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

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

site/templates/product.php

@@ -47,6 +47,9 @@
                         data-item-name="<?= $page->title()->html() ?>"
                         data-item-shippable="true"
                         data-item-weight="<?= $page->weight()->or(0) ?>"
+                        data-item-length="<?= $page->length()->or(0) ?>"
+                        data-item-width="<?= $page->width()->or(0) ?>"
+                        data-item-height="<?= $page->height()->or(0) ?>"
                         <?php
                         if($page->hasOptions()->toBool() && $page->optionValues()->isNotEmpty()):
                             $values = $page->optionValues()->split(',');