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>

assets/css/style.css

@@ -559,6 +559,7 @@ main {
   color: var(--color-txt);
 }
 #site-header .header-cart-btn {
+  font-family: var(--font);
   background: none;
   border: none;
   cursor: pointer;