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/call-status/MemberSpeaking.tsx

@@ -1,17 +1,14 @@
 import { Room } from 'matrix-js-sdk';
 import React from 'react';
 import { Box, Icon, Icons, Text } from 'folds';
-import { getMemberDisplayName } from '../../utils/room';
-import { getMxIdLocalPart } from '../../utils/matrix';
+import { getMemberName } from '../../utils/room';
 
 type MemberSpeakingProps = {
   room: Room;
   speakers: Set<string>;
 };
 export function MemberSpeaking({ room, speakers }: MemberSpeakingProps) {
-  const speakingNames = Array.from(speakers).map(
-    (userId) => getMemberDisplayName(room, userId) ?? getMxIdLocalPart(userId) ?? userId,
-  );
+  const speakingNames = Array.from(speakers).map((userId) => getMemberName(room, userId));
   return (
     <Box alignItems="Center" gap="100">
       <Icon size="100" src={Icons.Mic} filled />