Chat panel: Enter key to send

[siddhss5]

Viser's add_text on_update fires per keystroke, not on Enter. Need custom JS via add_html to capture Enter keypress and trigger the send callback. Low priority — Send button works.

[siddhss5]

Blocked on Viser upstream: viser#681. Viser's add_text fires on_update per keystroke, so Enter can't be distinguished from typing. The chat panel documents this limitation at the top of the file. Nothing to do on our side until Viser adds a submit event.