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/room-pin-menu/RoomPinMenu.tsx

@@ -40,12 +40,12 @@ import {
   UsernameBold,
 } from '../../../components/message';
 import { UserAvatar } from '../../../components/user-avatar';
-import { getMxIdLocalPart, mxcUrlToHttp } from '../../../utils/matrix';
+import { mxcUrlToHttp } from '../../../utils/matrix';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import {
   getEditedEvent,
   getMemberAvatarMxc,
-  getMemberDisplayName,
+  getMemberName,
   getStateEvent,
 } from '../../../utils/room';
 import { GetContentCallback, MessageEvent, StateEvent } from '../../../../types/matrix/room';
@@ -175,7 +175,7 @@ function PinnedMessage({
     );
 
   const sender = pinnedEvent.getSender()!;
-  const displayName = getMemberDisplayName(room, sender) ?? getMxIdLocalPart(sender) ?? sender;
+  const displayName = getMemberName(room, sender);
   const senderAvatarMxc = getMemberAvatarMxc(room, sender);
   const getContent = (() => pinnedEvent.getContent()) as GetContentCallback;