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

lib/package.json

@@ -10,7 +10,9 @@
     "src/"
   ],
   "scripts": {
-    "build": "rollup -c",
+    "build": "npm run build:js && npm run build:css",
+    "build:js": "rollup -c",
+    "build:css": "cp src/styles/insertr.css dist/insertr.css",
     "build:only": "rollup -c",
     "watch": "rollup -c -w",
     "dev": "rollup -c -w"