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

test/profiles/profile_activation_test.dart

@@ -118,6 +118,7 @@ class _ThrowingActiveProfileProvider extends ActiveProfileProvider {
     required super.registry,
     required super.plexHome,
     required super.connections,
+    required super.profileConnections,
     required super.storage,
     super.activeProfileIdWriter,
   });
@@ -515,6 +516,7 @@ void main() {
       registry: profiles,
       plexHome: plexHome,
       connections: connections,
+      profileConnections: profileConnections,
       storage: storage,
       activeProfileIdWriter: writer,
     );
@@ -623,6 +625,7 @@ Future<_Harness> _pumpHarness(
     registry: profiles,
     plexHome: plexHome,
     connections: connections,
+    profileConnections: profileConnections,
     storage: storage,
     activeProfileIdWriter: gateOwnerRestoreWrite || failNewerIdentityWrite ? activeProfileIdWriter : null,
   );