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/pages/client/ClientNonUIFeatures.tsx

@@ -27,13 +27,13 @@ import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { getDirectRoomPath, getHomeRoomPath, getInboxInvitesPath } from '../pathUtils';
 import { mDirectAtom } from '../../state/mDirectList';
 import {
-  getMemberDisplayName,
+  getMemberName,
   getNotificationType,
   getUnreadInfo,
   isNotificationEvent,
 } from '../../utils/room';
 import { NotificationType } from '../../../types/matrix/room';
-import { getMxIdLocalPart, mxcUrlToHttp } from '../../utils/matrix';
+import { mxcUrlToHttp } from '../../utils/matrix';
 import { useSelectedRoom } from '../../hooks/router/useSelectedRoom';
 import { useInboxNotificationsSelected } from '../../hooks/router/useInbox';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
@@ -501,7 +501,7 @@ function MessageNotifications() {
           roomAvatar: avatarMxc
             ? (mxcUrlToHttp(mx, avatarMxc, useAuthentication, 96, 96, 'crop') ?? undefined)
             : undefined,
-          username: getMemberDisplayName(room, sender) ?? getMxIdLocalPart(sender) ?? sender,
+          username: getMemberName(room, sender),
           roomId: room.roomId,
           eventId,
           body: (mEvent.getContent().body as string | undefined) ?? '',