Profile cards, nicknames, avatars, and custom status

## Summary
Implement profile cards, guild-local identity, avatars, nicknames, and custom status with Discord-like user-facing behavior.

## Scope
- Support guild-local nickname-first presentation for member identity.
- Add avatars and profile cards that surface the most relevant account and guild information.
- Add custom status and related lightweight presence expression.
- Make identity presentation consistent across sidebar, member directory, transcript, mentions, and popouts.

## Acceptance Criteria
- [ ] Members can set and display guild-relevant identity fields where allowed.
- [ ] Hover/click profile cards surface core identity and status information without leaving context.
- [ ] Avatar and nickname rendering stay consistent across all major surfaces.
- [ ] Permissions for editing identity are enforced correctly for self, moderators, and owners.

## Operational Notes
- RFChat should optimize for guild-local identity first because each deployment represents one community.
- Media/storage decisions for avatars must align with the durable storage strategy.

Start by mapping the existing identity, member, presentation, permissions, and storage entry points; the issue does not name specific files or tests. Trace how identity is currently rendered across the sidebar, member directory, transcript, mentions, and popouts. Done means the listed surfaces share guild-local identity, profile cards, avatars, status, and enforced editing permissions.

Written by the indexing model from the issue text.