Implement hybrid CSS architecture to fix white-on-white modal issue on sites with CSS resets

- Migrate from inline CSS to external insertr.css with cascade layer architecture
- Add CSS CDN serving capability (ServeInsertrCSS handler and /insertr.css route)
- Implement hybrid approach: @layer insertr for modern browsers + html body selectors for legacy browsers
- Remove scattered inline CSS from JavaScript modules for better maintainability
- Solve form element spacing conflicts with aggressive site CSS resets like '* {margin:0; padding:0}'
- Enable proper CSS caching and separation of concerns

justfile

@@ -69,7 +69,7 @@ check:
 build:
     npm run build
 
-# Build only the JavaScript library
+# Build only the JavaScript library (includes CSS)
 build-lib:
     npm run build:lib