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/thread/ThreadTimeline.tsx

@@ -32,7 +32,7 @@ import { useAtomValue, useSetAtom } from 'jotai';
 import { Badge, Box, Chip, Icon, Icons, Line, Scroll, Spinner, Text, color, config } from 'folds';
 import classNames from 'classnames';
 import { Opts as LinkifyOpts } from 'linkifyjs';
-import { eventWithShortcode, factoryEventSentBy, getMxIdLocalPart } from '../../../utils/matrix';
+import { eventWithShortcode, factoryEventSentBy } from '../../../utils/matrix';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { useVirtualPaginator, ItemRange } from '../../../hooks/useVirtualPaginator';
 import { useAlive } from '../../../hooks/useAlive';
@@ -58,7 +58,7 @@ import {
   decryptAllTimelineEvent,
   getEditedEvent,
   getEventReactions,
-  getMemberDisplayName,
+  getMemberName,
   getReactionContent,
   reactionOrEditEvent,
 } from '../../../utils/room';
@@ -503,7 +503,7 @@ export function ThreadTimeline({ room, thread, editor }: ThreadTimelineProps) {
       evt.preventDefault();
       const userId = evt.currentTarget.getAttribute('data-user-id');
       if (!userId) return;
-      const name = getMemberDisplayName(room, userId) ?? getMxIdLocalPart(userId) ?? userId;
+      const name = getMemberName(room, userId);
       editor.insertNode(
         createMentionElement(
           userId,
@@ -651,8 +651,7 @@ export function ThreadTimeline({ room, thread, editor }: ThreadTimelineProps) {
         const getContent = (() =>
           editedEvent?.getContent()['m.new_content'] ?? mEvent.getContent()) as GetContentCallback;
         const senderId = mEvent.getSender() ?? '';
-        const senderDisplayName =
-          getMemberDisplayName(room, senderId) ?? getMxIdLocalPart(senderId) ?? senderId;
+        const senderDisplayName = getMemberName(room, senderId);
         return (
           <RenderMessageContent
             displayName={senderDisplayName}