Invites and onboarding flow

## Summary
Implement invites and onboarding for a single RFChat deployment with behavior-first parity to Discord on web desktop first.

## Scope
- Create, list, revoke, and share invite links/codes for the guild.
- Support core invite constraints such as expiration and maximum uses.
- Provide an invite landing page that preserves context through sign-up or sign-in.
- Route accepted invites into the guild shell with clear invalid, expired, revoked, and exhausted-invite states.

## Acceptance Criteria
- [ ] Members with invite permission can create and revoke invites from the product UI.
- [ ] Invite visitors can sign up or sign in and still land in the invited guild context without losing the invite.
- [ ] Expired, revoked, and max-use invites are enforced server-side and explained clearly in the UI.
- [ ] Invite creation and use are recorded in a way moderators/admins can inspect later.

## Operational Notes
- Invite codes must be unguessable.
- Self-hosted deployments need controls to disable or restrict invites for private communities.

Start by mapping the existing guild, sign-up/sign-in, and moderator/admin flows against the issue's scope and acceptance criteria. Define how invite creation, use, revocation, expiration, limits, and private-deployment controls fit together. Done means the listed UI states, server-side enforcement, preserved guild context, and inspectable invite records all work end to end.

Written by the indexing model from the issue text.