Add @user, @channel, @here, and @Compass agent mentions
to the conversations module. TipTap Mention extension with
suggestion popup, message_mentions junction table for
efficient querying, push notifications for mentioned users
respecting notifyLevel preferences, and edit support with
mention re-extraction. Also fix crypto.randomUUID fallback
for non-secure contexts.
Co-authored-by: Nicholai <nicholaivogelfilms@gmail.com>
