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/RoomViewFollowing.tsx

@@ -15,8 +15,7 @@ import { Room } from 'matrix-js-sdk';
 import classNames from 'classnames';
 import FocusTrap from 'focus-trap-react';
 
-import { getMemberDisplayName } from '../../utils/room';
-import { getMxIdLocalPart } from '../../utils/matrix';
+import { getMemberName } from '../../utils/room';
 import * as css from './RoomViewFollowing.css';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useRoomLatestRenderedEvent } from '../../hooks/useRoomLatestRenderedEvent';
@@ -39,10 +38,7 @@ export const RoomViewFollowing = as<'div', RoomViewFollowingProps>(
     const latestEventReaders = useRoomEventReaders(room, latestEvent?.getId());
     const names = latestEventReaders
       .filter((readerId) => readerId !== mx.getUserId())
-      .map(
-        (readerId) =>
-          getMemberDisplayName(room, readerId) ?? getMxIdLocalPart(readerId) ?? readerId,
-      );
+      .map((readerId) => getMemberName(room, readerId));
 
     const eventId = latestEvent?.getId();