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-settings/RoomInsights.tsx

@@ -6,8 +6,8 @@ import { SequenceCard } from '../../components/sequence-card';
 import { useRoom } from '../../hooks/useRoom';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
-import { getMemberAvatarMxc, getMemberDisplayName } from '../../utils/room';
-import { getMxIdLocalPart, mxcUrlToHttp } from '../../utils/matrix';
+import { getMemberAvatarMxc, getMemberName } from '../../utils/room';
+import { mxcUrlToHttp } from '../../utils/matrix';
 import { UserAvatar } from '../../components/user-avatar';
 
 // ── Helpers ───────────────────────────────────────────────────────────────────
@@ -297,8 +297,7 @@ export function RoomInsights({ requestClose }: RoomInsightsProps) {
                   <SectionHeader label="Most Active Members" />
                   <Box direction="Column" gap="200">
                     {stats.top5.map(([userId, count], index) => {
-                      const displayName =
-                        getMemberDisplayName(room, userId) ?? getMxIdLocalPart(userId) ?? userId;
+                      const displayName = getMemberName(room, userId);
                       const avatarMxc = getMemberAvatarMxc(room, userId);
                       const avatarUrl = avatarMxc
                         ? (mxcUrlToHttp(mx, avatarMxc, useAuthentication, 32, 32, 'crop') ??