fix: resolve handleFileUpload ReferenceError while preserving upload code for future

- Restore handleFileUpload import in widget-library.js to fix ReferenceError
- Keep all upload functionality code intact for future development
- Disable upload features in demo.html with "Under Development" messaging
- Remove upload from feature button examples but preserve the code
- Style disabled upload demo with greyed-out appearance
- Fix demo page functionality while maintaining upload code for future fixes

src/widget-library.js

@@ -5,6 +5,7 @@ import { detectPageLanguage } from "./language-detection.js";
 import { getThemeColors } from "./ui-themes.js";
 import { openTTSModal } from "./tts-modal.js";
 import { openTranslateModal } from "./translate-modal.js";
+import { handleFileUpload } from "./file-upload.js";
 
 // Get USE_CUSTOM_STYLING from window or default
 const USE_CUSTOM_STYLING = window.UNCLOSEAI_CUSTOM_STYLING !== false;