custom.js is optional, and the page no longer demands it

custom.js is optional, and the page no longer demands it

index.php always loaded it, but custom.js holds the customisations of the
single site and may not be distributed: whoever installs from scratch got a
404 on every page load, on both branches of the layout.

The tag is now emitted only if the file is really there. The hook it uses
(custom_bedge) was already guarded on the javascript side, so without the
file the room behaves exactly as it always has.

Same approach as cookie_law.* and brisk_donate.txt: what belongs to a single
installation must not break the others.

Checked in the container: with the file the page references it and downloads
it (200), without the file the page is served complete, does not name it and
nginx logs no 404. Both occurrences were tried, the one on the entry page and
the one in the room.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014M1jiEq9cHdE5SE5j6vFuE