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/inbox/Notifications.tsx

@@ -28,7 +28,7 @@ import { Opts as LinkifyOpts } from 'linkifyjs';
 import { useAtomValue } from 'jotai';
 import { Page, PageContent, PageContentCenter, PageHeader } from '../../../components/page';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
-import { getMxIdLocalPart, mxcUrlToHttp } from '../../../utils/matrix';
+import { mxcUrlToHttp } from '../../../utils/matrix';
 import { InboxNotificationsPathSearchParams } from '../../paths';
 import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
 import { SequenceCard } from '../../../components/sequence-card';
@@ -36,7 +36,7 @@ import { RoomAvatar, RoomIcon } from '../../../components/room-avatar';
 import {
   getEditedEvent,
   getMemberAvatarMxc,
-  getMemberDisplayName,
+  getMemberName,
   getRoomAvatarUrl,
 } from '../../../utils/room';
 import { ScrollTopContainer } from '../../../components/scroll-top-container';
@@ -448,10 +448,7 @@ function RoomNotificationsGroupComp({
         {notifications.map((notification) => {
           const { event } = notification;
 
-          const displayName =
-            getMemberDisplayName(room, event.sender) ??
-            getMxIdLocalPart(event.sender) ??
-            event.sender;
+          const displayName = getMemberName(room, event.sender);
           const senderAvatarMxc = getMemberAvatarMxc(room, event.sender);
           const getContent = (() => event.content) as GetContentCallback;