feat(profiles): show the first linked connection's user picture

A local profile had no picture of its own and always fell back to
initials. It now borrows the user picture of the connection it was
linked to first — oldest Connection.createdAt, ties broken by
connection id, since the join table carries no creation time.

Jellyfin links resolve to /Users/{id}/Images/Primary, keyed by the
PrimaryImageTag now captured at authentication and refreshed from the
/Users/Me body checkHealth already fetches. That endpoint is anonymous
on every Jellyfin release, so the URL carries no api_key and the access
token stays out of the image cache key. Plex links resolve the Home
user the link points at against PlexHomeService's live cache, so no
account-level lookup is needed and the picture tracks Plex's own
refresh.

The picture is derived per snapshot and never written back onto a
Profile: ProfileDetailScreen upserts the model it holds, so a
persisted URL would go stale and outlive the connection it came from.
Plex Home profiles are untouched, including one whose Plex avatar is
unset — it keeps its initials rather than borrowing a lent connection's
picture.

close #1667

lib/main.dart

@@ -1472,6 +1472,7 @@ class _MainAppState extends State<MainApp> with WidgetsBindingObserver {
               registry: context.read<ProfileRegistry>(),
               plexHome: context.read<PlexHomeService>(),
               connections: context.read<ConnectionRegistry>(),
+              profileConnections: context.read<ProfileConnectionRegistry>(),
               storage: context.read<StorageService>(),
             );
             unawaited(provider.initialize());