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/LiveChip.tsx

@@ -20,8 +20,8 @@ import FocusTrap from 'focus-trap-react';
 import { Room } from 'matrix-js-sdk';
 import * as css from './styles.css';
 import { stopPropagation } from '../../utils/keyboard';
-import { getMemberAvatarMxc, getMemberDisplayName } from '../../utils/room';
-import { getMxIdLocalPart, mxcUrlToHttp } from '../../utils/matrix';
+import { getMemberAvatarMxc, getMemberName } from '../../utils/room';
+import { mxcUrlToHttp } from '../../utils/matrix';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
 import { UserAvatar } from '../../components/user-avatar';
@@ -77,8 +77,7 @@ export function LiveChip({ count, room, members }: LiveChipProps) {
                     {members.map((callMember) => {
                       const userId = callMember.sender;
                       if (!userId) return null;
-                      const name =
-                        getMemberDisplayName(room, userId) ?? getMxIdLocalPart(userId) ?? userId;
+                      const name = getMemberName(room, userId);
                       const avatarMxc = getMemberAvatarMxc(room, userId);
                       const avatarUrl = avatarMxc
                         ? (mxcUrlToHttp(mx, avatarMxc, useAuthentication, 96, 96) ?? undefined)