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/utils/room.ts

@@ -30,6 +30,7 @@ import {
   UnreadInfo,
 } from '../../types/matrix/room';
 import { getMutedThreads, ThreadNotificationsContent } from './threadNotifications';
+import { getMxIdLocalPart } from './matrix';
 
 export const getStateEvent = (
   room: Room,
@@ -405,6 +406,9 @@ export const getMemberDisplayName = (room: Room, userId: string): string | undef
   return name;
 };
 
+export const getMemberName = (room: Room, userId: string): string =>
+  getMemberDisplayName(room, userId) ?? getMxIdLocalPart(userId) ?? userId;
+
 export const getMemberSearchStr = (
   member: RoomMember,
   query: string,