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.

lib/connection/connection_registry.dart

@@ -1,7 +1,6 @@
 import 'dart:async';
 import 'dart:convert';
 
-import 'package:collection/collection.dart';
 import 'package:drift/drift.dart';
 
 import '../database/app_database.dart';
@@ -41,17 +40,6 @@ class ConnectionRegistry {
     return _rowToConnection(row);
   }
 
-  /// Returns the user's preferred connection — either the row marked
-  /// [Connections.isDefault], or the only row if exactly one exists, or
-  /// null when no connections are stored.
-  Future<Connection?> getDefault() async {
-    final rows = await _db.select(_db.connections).get();
-    if (rows.isEmpty) return null;
-    final flagged = rows.firstWhereOrNull((r) => r.isDefault);
-    final picked = flagged ?? (rows.length == 1 ? rows.single : null);
-    return picked == null ? null : _rowToConnection(picked);
-  }
-
   /// Insert or replace [connection]. If this is the first stored connection
   /// it is automatically marked default; re-upserting an existing row keeps
   /// the row's current `isDefault` (so token/metadata refreshes don't clear