fix(presence): Do Not Disturb is real "busy" presence and keeps your status (#283)

DND was sent as presence unavailable with the literal status text
"dnd": it replaced your status, and other clients showed "dnd". Where
the server supports MSC3026 (busy_presence), DND now sends presence
org.matrix.msc3026.busy and leaves the status alone (Synapse keeps
busy across /sync set_presence=online). Busy renders as red "Do Not
Disturb" with the real status line; the legacy unavailable + "dnd"
encoding is still recognised and still used on servers without it.
Status saves / clears no longer downgrade busy. The SDK rejects
non-spec presences, so busy goes through a direct authed PUT.

Fixes #283

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/components/presence/PresenceRingAvatar.tsx

@@ -4,6 +4,7 @@ import { Presence, useUserPresence } from '../../hooks/useUserPresence';
 
 function presenceRingColor(presence: Presence | undefined, status?: string): string | null {
   if (!presence || presence === Presence.Offline) return null;
+  if (presence === Presence.Busy) return color.Critical.Main; // [Gitea #283] DND
   if (presence === Presence.Unavailable) {
     return status === 'dnd' ? color.Critical.Main : color.Warning.Main;
   }