feat: implement cross-platform file upload bridge for Flutter WebView integration

src/lib/webview-actions.ts

@@ -49,7 +49,10 @@ export interface DownloadFileOptions {
  * ```
  */
 export function downloadFile(options: DownloadFileOptions): boolean {
-  return postActionToFlutter("download_file", options as Record<string, unknown>);
+  return postActionToFlutter(
+    "download_file",
+    options as unknown as Record<string, unknown>,
+  );
 }
 
 // ─── copy_to_clipboard ──────────────────────────────────
@@ -142,5 +145,8 @@ export interface UploadFileOptions {
  * ```
  */
 export function uploadFile(options: UploadFileOptions): boolean {
-  return postActionToFlutter("upload_file", options as Record<string, unknown>);
+  return postActionToFlutter(
+    "upload_file",
+    options as unknown as Record<string, unknown>,
+  );
 }