i found 2 other posts with the same problem:
https://itch.io/t/1971050/unblocking-clipboard-api
https://itch.io/t/2104659/iframe-cant-read-from-clipboard
this problem can be partially solved on the application side.
unity has plugin which allows to enable copy-paste to both sides
in case of godot - copying to clipboard can be enabled by javascript callback. and NO way to copy into an input/textarea with those iframe limitations
please fix it. it's just one line change from your side. and in my first post i mentioned which additional attributes will allow to enable it