chore: add system docs, specs, and misc updates from other sessions

- Add complete docs/systems/ reference (18 system docs)
- Add federation relay status doc and prior spec/plan docs
- Remove superseded docs/federation-dm-s2s.md (replaced by docs/systems/federation.md)
- CLAUDE.md updates
- Minor fixes in social.ts, types.ts, AddDmMemberModal, NewDmModal, UserSettings

packages/shared/src/types.ts

@@ -797,6 +797,7 @@ export interface FederationGroupPayload {
 }
 
 export interface FederationRelayProfileSnapshot {
+  username?: string | null;
   displayName?: string | null;
   avatar?: string | null;
   avatarColor?: string | null;