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

config/_default/params.toml

@@ -29,6 +29,8 @@ disableTextInHeader = false
 # defaultFeaturedImage = "IMAGE.jpg" # used as default for featured images in all articles
 # defaultSocialImage = "/android-chrome-512x512.png" # used as default for social media sharing (Open Graph and Twitter)
 hotlinkFeatureImage = false
+# Thumbnail aspect ratio as a number (e.g., "1.5", "1.0", "1.777")
+# thumbnailAspectRatio = "1.5"
 # imagePosition = "50% 50%"
 
 # highlightCurrentMenuArea = true