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>

.claude/settings.local.json

@@ -10,7 +10,8 @@
       "Bash(grep:*)",
       "Bash(npm run build:*)",
       "Bash(php test-shopify.php:*)",
-      "WebFetch(domain:getkirby.com)"
+      "WebFetch(domain:getkirby.com)",
+      "WebFetch(domain:forum.getkirby.com)"
     ]
   }
 }