Fix mobile hamburger menu, designs page template, product tab styling, and stock field (REPA-78)

- Add mobile hamburger toggle button and full-screen nav drawer (≤1024px)
- Fix product tab styles: complete tab CSS without relying on WC parent selectors; explicit wc-single-product enqueue on product pages
- Fix STOCK field: conditionally hide when get_stock_quantity() returns null instead of showing '— units'
- Add page-designs.php template + template_include filter so /designs/ uses themed layout instead of raw WooCommerce output
- Align nav/footer terminology: 'Canvas' → 'Wall Decor' in nav fallback; correct broken category URL slugs (mugs→drinkware, canvas→wall-decor, planners→stationery, blog→the-memo); add 'Bulk / HR' to mobile drawer

Co-Authored-By: Paperclip <noreply@paperclip.ing>

theme/woocommerce/single-product.php

@@ -147,10 +147,12 @@ $stock_color = $stock === 'instock' ? 'var(--olive-dark)' : ($stock === 'onbacko
                 <div class="rar-shipping-item__key">Origin</div>
                 <div class="rar-shipping-item__val">Cleveland, OH</div>
             </div>
+            <?php $stock_qty = $product->get_stock_quantity(); if ($stock_qty !== null): ?>
             <div class="rar-shipping-item">
                 <div class="rar-shipping-item__key">Stock</div>
-                <div class="rar-shipping-item__val"><?php echo esc_html($product->get_stock_quantity() ?? '—'); ?> units</div>
+                <div class="rar-shipping-item__val"><?php echo esc_html($stock_qty); ?> units</div>
             </div>
+            <?php endif; ?>
         </div>
     </div>
 </div>