Convert CaliBlur background images to WebP format for 97% size reduction

- Convert all PNG background images to WebP (2.2MB → 60KB total)
- Update CSS references in caliBlur.css to use .webp files
- Improves page load performance and reduces bandwidth usage

Author: crocodilestick

CONTRIBUTORS

@@ -1,7 +1,7 @@
 CONTRIBUTORS
 
 This file is automatically generated. DO NOT EDIT MANUALLY.
-Generated on: 2025-09-14T13:56:02.471203Z
+Generated on: 2025-09-14T14:20:19.788061Z
 
 Upstream project: https://github.com/janeczku/calibre-web
 Fork project (Calibre-Web Automated, since 2024): https://github.com/crocodilestick/calibre-web-automated