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/RoomTimeline.tsx

@@ -81,6 +81,7 @@ import {
   getEventReactions,
   getLatestEditableEvt,
   getMemberDisplayName,
+  getMemberName,
   getReactionContent,
   isMembershipChanged,
   reactionOrEditEvent,
@@ -1007,7 +1008,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
         console.warn('Button should have "data-user-id" attribute!');
         return;
       }
-      const name = getMemberDisplayName(room, userId) ?? getMxIdLocalPart(userId) ?? userId;
+      const name = getMemberName(room, userId);
       editor.insertNode(
         createMentionElement(
           userId,
@@ -1106,8 +1107,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
           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 (
           <Message
@@ -1290,8 +1290,7 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
                     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}