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- Remove superseded docs/federation-dm-s2s.md (replaced by docs/systems/federation.md)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/server/src/routes/social.ts

@@ -18,6 +18,7 @@ import { sanitizeUser } from '../utils/sanitize.js';
 
 function buildProfileSnapshot(user: typeof schema.users.$inferSelect): FederationRelayProfileSnapshot {
   return {
+    username: user.username ?? null,
     displayName: user.displayName ?? null,
     avatar: user.avatar ?? null,
     avatarColor: user.avatarColor ?? null,