Improve Open Graph meta tags with default description

Add default descriptions for social sharing cards:
- FR: "Boutique de Index, ONG d'investigation indépendante"
- EN: "Index shop, independent investigative NGO"

Changes:
- Add language-specific default descriptions
- Fix description excerpt handling
- Add OG image generation helper (create-og-image.html)
- Use favicon as temporary OG image (TODO: create proper 1200x630 image)

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

site/snippets/seo.php

@@ -3,16 +3,30 @@
  * SEO meta tags
  */
 
+// Language
+$lang = $kirby->language()->code();
+
 // Basic meta
 $title = $page->customTitle()->or($page->title())->value();
 $siteName = 'Index.ngo';
 $fullTitle = $title . ' | ' . $siteName;
-$description = $page->metaDescription()->or($page->description())->excerpt(160);
-$url = $page->url();
-$image = $page->image() ? $page->image()->url() : url('assets/og-image.jpg');
 
-// Language
-$lang = $kirby->language()->code();
+// Default descriptions by language
+$defaultDescriptionFr = 'Boutique de Index, ONG d\'investigation indépendante';
+$defaultDescriptionEn = 'Index shop, independent investigative NGO';
+$defaultDescription = $lang == 'en' ? $defaultDescriptionEn : $defaultDescriptionFr;
+
+$description = $page->metaDescription()->or($page->description())->value();
+if ($description) {
+    $description = excerpt($description, 160);
+} else {
+    $description = $defaultDescription;
+}
+
+$url = $page->url();
+// Use product image if available, otherwise use default OG image
+// TODO: Create assets/og-logo.png (1200x630px) with Index logo + description
+$image = $page->image() ? $page->image()->url() : url('assets/favicon.png');
 ?>
 
 <!-- Basic Meta Tags -->