fix(composer): scheduling keeps formatting, mentions and the reply relation

The schedule modal took only a plain body and rebuilt {body, msgtype},
dropping formatted_body, m.mentions and m.relates_to. It now receives
the full IContent; an unedited body is sent verbatim, an edited body
drops the now-stale formatted_body but keeps mentions and the reply/
thread relation. Reschedule from the tray preserves them too.
Unit-tested (mergeScheduledBody).

Fixes #36

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room/ScheduledMessagesTray.tsx

@@ -225,7 +225,7 @@ export function ScheduledMessagesTray({ roomId }: ScheduledMessagesTrayProps) {
       {editing && (
         <ScheduleMessageModal
           roomId={roomId}
-          initialBody={typeof editing.content.body === 'string' ? editing.content.body : ''}
+          initialContent={editing.content}
           initialSendAt={editing.sendAt}
           title="Edit scheduled message"
           submitLabel="Reschedule"