Server settings surface

## Summary
Implement the main server settings surface as the administrative control plane for a single RFChat deployment.

## Scope
- Provide a coherent settings IA for branding, safety defaults, notifications, moderation, and other server-level controls.
- Give admins clear entry points into specialized management areas such as channels, roles, invites, audit, bots, and media.
- Keep the settings experience behavior-first and comprehensible for self-hosting admins, not only Discord veterans.
- Support permission-aware visibility so members see only relevant settings.

## Acceptance Criteria
- [ ] Server settings are organized into clear tabs or sections with predictable navigation.
- [ ] Server-level controls and specialized management tools are discoverable from one coherent settings surface.
- [ ] Unauthorized users do not see privileged settings areas.
- [ ] The settings shell scales as additional admin features are added without becoming structurally inconsistent.

## Operational Notes
- Because each deployment is one guild, server settings are a primary product surface, not a niche admin modal.
- Self-hosted admin UX should prefer clarity and auditability over clever density.