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/RoomInput.tsx

@@ -65,7 +65,6 @@ import {
   TUploadContent,
   encryptFile,
   getImageInfo,
-  getMxIdLocalPart,
   mxcUrlToHttp,
   tryDeleteMxcContent,
 } from '../../utils/matrix';
@@ -111,7 +110,7 @@ import {
   getImageMsgContent,
   getVideoMsgContent,
 } from './msgContent';
-import { getMemberDisplayName, getMentionContent, trimReplyFromBody } from '../../utils/room';
+import { getMemberName, getMentionContent, trimReplyFromBody } from '../../utils/room';
 import { CommandAutocomplete } from './CommandAutocomplete';
 import { Command, SHRUG, TABLEFLIP, UNFLIP, useCommands } from '../../hooks/useCommands';
 import { mobileOrTablet } from '../../utils/user-agent';
@@ -993,11 +992,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
                       userColor={replyUsernameColor}
                       username={
                         <Text size="T300" truncate>
-                          <b>
-                            {getMemberDisplayName(room, replyDraft.userId) ??
-                              getMxIdLocalPart(replyDraft.userId) ??
-                              replyDraft.userId}
-                          </b>
+                          <b>{getMemberName(room, replyDraft.userId)}</b>
                         </Text>
                       }
                     >