gh-149144: Use decodeURIComponent() for UTF-8 support in js_output()

[sethmlarson]

Related to #90309. Fixes a bug in the security fix #148848

The JavaScript function atob() only outputs strings in latin-1, not in utf-8. decodeURIComponent outputs to UTF-8. Tested in Firefox and Chrome and the function decodeURIComponent is supported by IE 5.5+.

Python:

>>> import urllib.parse as u
>>> u.quote('Ł/😀', safe='', encoding='utf-8')
'%C5%81%2F%F0%9F%98%80'

JavaScript (Firefox, Chrome):

>>> decodeURIComponent("%C5%81%2F%F0%9F%98%80")
<<< "Ł/😀" 

• Issue: CVE-2026-6019 fix does not handle non-ascii chars correctly #149144