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/test_helpers/profile_stack.dart

@@ -59,7 +59,13 @@ class ProfileStack {
       profileConnections: profileConnections,
       profiles: profiles,
       plexHome: plexHome,
-      active: ActiveProfileProvider(registry: profiles, plexHome: plexHome, connections: connections, storage: storage),
+      active: ActiveProfileProvider(
+        registry: profiles,
+        plexHome: plexHome,
+        connections: connections,
+        profileConnections: profileConnections,
+        storage: storage,
+      ),
       storage: storage,
       ownsDatabase: db == null,
     );