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/pages/client/inbox/Invites.tsx

@@ -37,7 +37,7 @@ import {
   bannedInRooms,
   getCommonRooms,
   getDirectRoomAvatarUrl,
-  getMemberDisplayName,
+  getMemberName,
   getRoomAvatarUrl,
   getStateEvent,
   isDirectInvite,
@@ -48,7 +48,6 @@ import { RoomAvatar } from '../../../components/room-avatar';
 import {
   addRoomIdToMDirect,
   declineInvite,
-  getMxIdLocalPart,
   guessDmRoomUserId,
   rateLimitedActions,
 } from '../../../utils/matrix';
@@ -107,9 +106,7 @@ const makeInviteData = (mx: MatrixClient, room: Room, useAuthentication: boolean
   const content = memberEvent?.getContent();
   const senderId = memberEvent?.getSender();
 
-  const senderName = senderId
-    ? (getMemberDisplayName(room, senderId) ?? getMxIdLocalPart(senderId) ?? senderId)
-    : undefined;
+  const senderName = senderId ? getMemberName(room, senderId) : undefined;
   const inviteTs = memberEvent?.getTs();
   const reason =
     content && 'reason' in content && typeof content.reason === 'string'