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/components/editor/autocomplete/UserMentionAutocomplete.tsx

@@ -16,7 +16,7 @@ import { onTabPress } from '../../../utils/keyboard';
 import { createMentionElement, moveCursor, replaceWithElement } from '../utils';
 import { useKeyDown } from '../../../hooks/useKeyDown';
 import { getMxIdLocalPart, getMxIdServer, isUserId } from '../../../utils/matrix';
-import { getMemberDisplayName, getMemberSearchStr } from '../../../utils/room';
+import { getMemberName, getMemberSearchStr } from '../../../utils/room';
 import { UserAvatar } from '../../user-avatar';
 import { useMediaAuthentication } from '../../../hooks/useMediaAuthentication';
 import { Membership } from '../../../../types/matrix/room';
@@ -140,8 +140,7 @@ export function UserMentionAutocomplete({
     });
   });
 
-  const getName = (member: RoomMember) =>
-    getMemberDisplayName(room, member.userId) ?? getMxIdLocalPart(member.userId) ?? member.userId;
+  const getName = (member: RoomMember) => getMemberName(room, member.userId);
 
   return (
     <AutocompleteMenu headerContent={<Text size="L400">Mentions</Text>} requestClose={requestClose}>