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/message/MessageEditor.tsx

@@ -53,11 +53,10 @@ import { AsyncStatus, useAsyncCallback } from '../../../hooks/useAsyncCallback';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import {
   getEditedEvent,
-  getMemberDisplayName,
+  getMemberName,
   getMentionContent,
   trimReplyFromFormattedBody,
 } from '../../../utils/room';
-import { getMxIdLocalPart } from '../../../utils/matrix';
 import { mobileOrTablet } from '../../../utils/user-agent';
 import { useComposingCheck } from '../../../hooks/useComposingCheck';
 
@@ -75,9 +74,7 @@ export const MessageEditor = as<'div', MessageEditorProps>(
     // Accessible name for the edit textbox so screen readers announce which
     // message is being edited (a11y, P3-4).
     const editSenderId = mEvent.getSender();
-    const editSenderName = editSenderId
-      ? (getMemberDisplayName(room, editSenderId) ?? getMxIdLocalPart(editSenderId) ?? editSenderId)
-      : '';
+    const editSenderName = editSenderId ? getMemberName(room, editSenderId) : '';
     const [enterForNewline] = useSetting(settingsAtom, 'enterForNewline');
     const [globalToolbar] = useSetting(settingsAtom, 'editorToolbar');
     const [isMarkdown] = useSetting(settingsAtom, 'isMarkdown');