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/ScheduleMessageModal.tsx

@@ -31,8 +31,13 @@ import {
 
 interface ScheduleMessageModalProps {
   roomId: string;
-  /** Pre-fill the message body from the composer. Pass null/undefined to open blank. */
-  initialBody?: string;
+  /**
+   * Pre-fill from the composer (or the message being rescheduled). The textarea
+   * edits only `body`; the rest of the content (mentions, reply/thread relation,
+   * formatting) rides along — see `mergeScheduledBody`. Pass null/undefined to
+   * open blank.
+   */
+  initialContent?: IContent | null;
   /** Pre-fill the date/time pickers (Unix ms) — used when editing/rescheduling. */
   initialSendAt?: number;
   /** Header title; defaults to "Schedule Message". */
@@ -43,6 +48,23 @@ interface ScheduleMessageModalProps {
   onClose: () => void;
 }
 
+/**
+ * [Gitea #36] Build the content to schedule from the pre-filled composer content
+ * and the (possibly edited) textarea body. Previously the modal rebuilt a bare
+ * `{ body, msgtype }`, silently dropping `formatted_body`, `m.mentions` and the
+ * reply/thread relation. Now: an unedited body sends the content verbatim; an
+ * edited body replaces `body` and drops `format`/`formatted_body` (they would be
+ * stale) but keeps mentions and the relation.
+ */
+export function mergeScheduledBody(initial: IContent | null | undefined, body: string): IContent {
+  const trimmed = body.trim();
+  if (!initial) return { body: trimmed, msgtype: 'm.text' };
+  const initialBody = typeof initial.body === 'string' ? initial.body : '';
+  if (trimmed === initialBody.trim()) return initial;
+  const { format: _format, formatted_body: _formattedBody, ...rest } = initial;
+  return { ...rest, msgtype: initial.msgtype ?? 'm.text', body: trimmed };
+}
+
 function formatRelativeTime(ms: number): string {
   const totalSeconds = Math.floor(ms / 1000);
   const hours = Math.floor(totalSeconds / 3600);
@@ -55,7 +77,7 @@ function formatRelativeTime(ms: number): string {
 
 export function ScheduleMessageModal({
   roomId,
-  initialBody,
+  initialContent,
   initialSendAt,
   title = 'Schedule Message',
   submitLabel = 'Schedule',
@@ -64,7 +86,9 @@ export function ScheduleMessageModal({
 }: ScheduleMessageModalProps) {
   const modalStyle = useModalStyle(400);
   const mx = useMatrixClient();
-  const [messageText, setMessageText] = useState(initialBody ?? '');
+  const [messageText, setMessageText] = useState(
+    typeof initialContent?.body === 'string' ? initialContent.body : '',
+  );
   const [submitting, setSubmitting] = useState(false);
   const [error, setError] = useState<string | null>(null);
 
@@ -128,7 +152,7 @@ export function ScheduleMessageModal({
       return;
     }
 
-    const content: IContent = { body: messageText.trim(), msgtype: 'm.text' };
+    const content = mergeScheduledBody(initialContent, messageText);
     setError(null);
     setSubmitting(true);
     try {