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/reaction-viewer/ReactionViewer.tsx

@@ -16,7 +16,7 @@ import {
 } from 'folds';
 import { MatrixEvent, Room, RoomMember } from 'matrix-js-sdk';
 import { Relations } from 'matrix-js-sdk/lib/models/relations';
-import { getMemberDisplayName } from '../../../utils/room';
+import { getMemberName } from '../../../utils/room';
 import { eventWithShortcode, getMxIdLocalPart } from '../../../utils/matrix';
 import * as css from './ReactionViewer.css';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
@@ -67,8 +67,7 @@ export const ReactionViewer = as<'div', ReactionViewerProps>(
       }
     };
 
-    const getName = (member: RoomMember) =>
-      getMemberDisplayName(room, member.userId) ?? getMxIdLocalPart(member.userId) ?? member.userId;
+    const getName = (member: RoomMember) => getMemberName(room, member.userId);
 
     const getReactionsForKey = (key: string): MatrixEvent[] => {
       const reactSet = reactions.find(([k]) => k === key)?.[1];