refactor(profiles): remove dead registry surface

Delete unused registry members (ProfileConnectionRegistry.insertIfAbsent /
removeAllForProfile, ProfileRegistry.reorder, ProfilesView.countFor,
ConnectionRegistry.getDefault) and their orphan tests, refreshing the stale
docs that named removeAllForProfile as the profile-delete cleanup path.

Repair the "one default per profile" join-row invariant: the connectionId FK
cascade (foreign_keys=ON) silently drops a profile's default row while its
other rows survive, leaving it defaultless. Add promoteMissingDefaults, share
a deterministic re-promotion helper with remove(), and re-promote in
removeAllForConnection.

test/database/app_database_test.dart

@@ -58,10 +58,10 @@ class _AppDatabaseTestSuite {
 
       test('ProfileConnections has no profile_id FK (virtual plex_home profiles)', () async {
         // v20 dropped the profile_id FK so virtual Plex Home profiles can
-        // persist join rows without a parent `profiles` row. The two
-        // profile-delete sites (profile_detail_screen, profile_switch_screen)
-        // call ProfileConnectionRegistry.removeAllForProfile manually before
-        // deleting the profile, so the cascade isn't needed.
+        // persist join rows without a parent `profiles` row. Profile deletion
+        // instead cleans up join rows explicitly (via the teardown flow's
+        // removeAllProfileConnectionsAndCleanup) before deleting the profile,
+        // so the cascade isn't needed.
         final now = DateTime.now().millisecondsSinceEpoch;
         await db
             .into(db.connections)