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/CallEmbedProvider.tsx

@@ -59,7 +59,7 @@ import { useTheme, ThemeKind } from '../hooks/useTheme';
 import { useReducedMotion } from '../hooks/useReducedMotion';
 import { useSetting } from '../state/hooks/settings';
 import { settingsAtom } from '../state/settings';
-import { getStateEvent, getStateEvents, getMemberDisplayName } from '../utils/room';
+import { getStateEvent, getStateEvents, getMemberName } from '../utils/room';
 import { StateEvent } from '../../types/matrix/room';
 import { getPowersLevelFromMatrixEvent } from '../hooks/usePowerLevels';
 import { getRoomCreatorsForRoomId } from '../hooks/useRoomCreators';
@@ -161,9 +161,7 @@ function IncomingCall({ dm, info, onIgnore, onAnswer, onReject }: IncomingCallPr
           <Dialog style={{ maxWidth: toRem(324) }}>
             <Box style={{ padding: config.space.S400 }} direction="Column" gap="700">
               <Text size="T200" align="Center">
-                {getMemberDisplayName(info.room, info.sender) ??
-                  getMxIdLocalPart(info.sender) ??
-                  info.sender}
+                {getMemberName(info.room, info.sender)}
               </Text>
               <Box direction="Column" gap="500" alignItems="Center">
                 <Box shrink="No">
@@ -316,8 +314,7 @@ function IncomingCallBanner({ dm, info, onIgnore, onAnswer, onReject }: Incoming
     return () => clearTimeout(id);
   }, [info.senderTs, info.lifetime, onIgnore]);
 
-  const callerName =
-    getMemberDisplayName(info.room, info.sender) ?? getMxIdLocalPart(info.sender) ?? info.sender;
+  const callerName = getMemberName(info.room, info.sender);
 
   return (
     <Box
@@ -453,8 +450,7 @@ function IncomingCallListener({ callEmbed, joined }: IncomingCallListenerProps)
           decliner !== mx.getSafeUserId() &&
           callEmbed?.roomId === room.roomId
         ) {
-          const declinerName =
-            getMemberDisplayName(room, decliner) ?? getMxIdLocalPart(decliner) ?? decliner;
+          const declinerName = getMemberName(room, decliner);
           setToast({
             id: `rtc-decline-${event.getId() ?? decliner}`,
             displayName: declinerName,