Add variable product support to product selector

BREAKING CHANGE: Variable products now supported in composable products.

Changes:
- Modified get_available_products() to detect variable products
- Variable products now expand to show all available variations
- Each variation is checked individually for stock and purchasability
- Simple products continue to work as before

This allows customers to select specific variations (e.g., "T-Shirt - Red - Large")
instead of just seeing "T-Shirt" which wasn't selectable.

Fixes the issue where products weren't showing because all products
in the category/tag were variable products.

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

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

includes/Product_Type.php

@@ -163,7 +163,22 @@ class Product_Type extends \WC_Product {
         if ($query->have_posts()) {
             foreach ($query->posts as $post) {
                 $product = wc_get_product($post->ID);
-                if ($product && $product->is_in_stock() && $product->is_purchasable()) {
+
+                if (!$product) {
+                    continue;
+                }
+
+                // Handle variable products by including their variations
+                if ($product->is_type('variable')) {
+                    $variations = $product->get_available_variations();
+                    foreach ($variations as $variation_data) {
+                        $variation = wc_get_product($variation_data['variation_id']);
+                        if ($variation && $variation->is_in_stock() && $variation->is_purchasable()) {
+                            $products[] = $variation;
+                        }
+                    }
+                } elseif ($product->is_in_stock() && $product->is_purchasable()) {
+                    // Simple and other product types
                     $products[] = $product;
                 }
             }