DP18: add getMemberName helper and dedup name-fallback sites

Add a pure `getMemberName(room, userId): string` helper in utils/room.ts
(= getMemberDisplayName ?? getMxIdLocalPart ?? userId) and replace the
inline `getMemberDisplayName(room, id) ?? getMxIdLocalPart(id) ?? id`
fallback across the codebase. No behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/message/Message.tsx

@@ -53,9 +53,9 @@ import {
   canEditEvent,
   getEventEdits,
   getMemberAvatarMxc,
-  getMemberDisplayName,
+  getMemberName,
 } from '../../../utils/room';
-import { getMxIdLocalPart, mxcUrlToHttp } from '../../../utils/matrix';
+import { mxcUrlToHttp } from '../../../utils/matrix';
 import { messageAriaLabel } from '../../../utils/a11y';
 import { MessageLayout, MessageSpacing } from '../../../state/settings';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
@@ -809,8 +809,7 @@ export const Message = React.memo(
       const [remindOpen, setRemindOpen] = useState(false);
       const { addBookmark, removeBookmark, isBookmarked } = useBookmarks();
 
-      const senderDisplayName =
-        getMemberDisplayName(room, senderId) ?? getMxIdLocalPart(senderId) ?? senderId;
+      const senderDisplayName = getMemberName(room, senderId);
       const senderAvatarMxc = getMemberAvatarMxc(room, senderId);
 
       const tagColor = memberPowerTag?.color