fix(build): stop register.html alias clobbering REGISTER.html on Windows

On case-insensitive filesystems the lowercase register.html redirect
alias resolves to REGISTER.html itself, so local builds overwrote the
real registration page with a self-redirect stub - which is what had
been committed. Skip the file alias when the two paths resolve to the
same file (Linux CI still writes it) and restore the real committed
html/REGISTER.html.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: GitHub Workshop Bot