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/message-search/SearchResultGroup.tsx

@@ -12,7 +12,7 @@ import {
   makeMentionCustomProps,
   renderMatrixMention,
 } from '../../plugins/react-custom-html-parser';
-import { getMxIdLocalPart, mxcUrlToHttp } from '../../utils/matrix';
+import { mxcUrlToHttp } from '../../utils/matrix';
 import { useMatrixEventRenderer } from '../../hooks/useMatrixEventRenderer';
 import { GetContentCallback, MessageEvent, StateEvent } from '../../../types/matrix/room';
 import {
@@ -31,7 +31,7 @@ import { Image } from '../../components/media';
 import { ImageViewer } from '../../components/image-viewer';
 import * as customHtmlCss from '../../styles/CustomHtml.css';
 import { RoomAvatar, RoomIcon } from '../../components/room-avatar';
-import { getMemberAvatarMxc, getMemberDisplayName, getRoomAvatarUrl } from '../../utils/room';
+import { getMemberAvatarMxc, getMemberName, getRoomAvatarUrl } from '../../utils/room';
 import { ResultItem } from './useMessageSearch';
 import { SequenceCard } from '../../components/sequence-card';
 import { UserAvatar } from '../../components/user-avatar';
@@ -220,10 +220,7 @@ export function SearchResultGroup({
         {items.map((item) => {
           const { event } = item;
 
-          const displayName =
-            getMemberDisplayName(room, event.sender) ??
-            getMxIdLocalPart(event.sender) ??
-            event.sender;
+          const displayName = getMemberName(room, event.sender);
           const senderAvatarMxc = getMemberAvatarMxc(room, event.sender);
 
           const relation = event.content['m.relates_to'];