feat: global thumbnail aspect ratio configuration

- Add thumbnailAspectRatio site parameter (default '1.5')
- Use decimal values instead of fractions for Hugo compatibility
- Sanitize user input to extract only numeric characters
- Apply to article cards, related cards, and term cards
- Update both source and compiled CSS

Closes #2839

assets/css/compiled/main.css

@@ -4676,10 +4676,12 @@ pre {
 }
 .thumbnail_card {
   min-width: 300px;
-  height: 200px;
+  aspect-ratio: var(--thumbnail-aspect-ratio, 1.5);
+  height: auto;
 }
 .thumbnail_card_related {
-  height: 150px;
+  aspect-ratio: var(--thumbnail-aspect-ratio, 1.5);
+  height: auto;
 }
 .thumbnail {
   width: 300px;