Direct messages and group DMs

## Summary
Implement direct messages and group DMs with Discord-like core behavior adapted to a single-guild deployment.

## Scope
- Support one-to-one DMs and small group DMs.
- Reuse core chat behaviors users already expect: unread state, mentions, replies, reactions, pins, and search where applicable.
- Keep DMs distinct from guild channels in navigation and permissions.
- Defer the broader social graph beyond what is necessary to make DMs usable.

## Acceptance Criteria
- [ ] Users can start, receive, and continue one-to-one DMs.
- [ ] Users can create and use small group DMs with sane membership rules.
- [ ] DM navigation, unread state, and notification behavior are coherent and separate from guild channels.
- [ ] DM privacy boundaries are enforced server-side.

## Operational Notes
- Because RFChat is single-guild-per-deployment, DMs should optimize for that community model rather than global cross-server identity.
- Social-layer expansion beyond core DM usability should stay in separate issues.

No files, tests, or entry points are named. Start by mapping the existing guild-channel navigation, chat behaviors, and server-side permission boundaries before choosing where DMs fit. Done means one-to-one and small group DMs work, remain distinct from guild channels, preserve coherent unread and notification behavior, and enforce privacy server-side.

Written by the indexing model from the issue text.