Forum-2 Reply to threads

Inside: forum/thread/[threadId]

we should add:

Reply input.
Submit reply.
Save reply to Supabase.
Associate it with:
thread_id
user_id
creation timestamp.
Refresh/update the thread so the new reply appears immediately.

I'd also make sure the database structure supports replies properly rather than treating them as another kind of thread.

Start at forum/thread/[threadId] and inspect how threads are loaded and updated, then review the existing Supabase database structure and user associations. The work is done when users can submit replies linked to the thread and user with a creation timestamp, and the new reply appears immediately after submission.

Written by the indexing model from the issue text.