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/hooks/useUserPresence.ts

@@ -6,6 +6,8 @@ export enum Presence {
   Online = 'online',
   Unavailable = 'unavailable',
   Offline = 'offline',
+  // [Gitea #283] MSC3026 busy: Lotus's Do Not Disturb.
+  Busy = 'org.matrix.msc3026.busy',
 }
 
 export type UserPresence = {
@@ -129,6 +131,7 @@ export const usePresenceLabel = (): Record<Presence, string> =>
       [Presence.Online]: 'Online',
       [Presence.Unavailable]: 'Idle',
       [Presence.Offline]: 'Offline',
+      [Presence.Busy]: 'Do Not Disturb',
     }),
     [],
   );