Fix multilingual routing for virtual product pages

Use site()->visit() to properly set language context for virtual pages.
This ensures UI translations and language-specific content work correctly
on both /slug (French) and /en/slug (English) routes.

Changes:
- Add site()->visit($page, $lang) in routes to set page language
- Create product controller for language detection
- Fix add-to-cart button to update text in .txt div instead of button
- Remove broken hooks approach

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

site/controllers/product.php

@@ -0,0 +1,11 @@
+<?php
+
+return function ($page, $kirby) {
+    $shopifyHandle = $page->shopifyHandle()->or($page->slug())->value();
+    $language = $kirby->language()->code();
+
+    return [
+        'shopifyHandle' => $shopifyHandle,
+        'language' => $language
+    ];
+};