fix(app): stop probing the control plane while disconnected

VpnConnection._init() started a 30 s /health poll from the constructor
with no lifecycle gating, so it ran for the life of the app process -
backgrounded, and whether or not the user ever connected. The app is
excluded from its own tunnel, so those requests left in the clear. A
periodic request that fires while the user is deliberately not connected
is an avoidable fingerprint, and rule 60 already ruled probes out:
reachability is meant to be passive.

All it bought was the status dot before the first connect, and it got
that wrong in both directions - "Offline" on a transient resolver
failure against a server that was fine.

Reachability now comes from a session sync that succeeded. serverHealthy
is isConnected && hasSyncedOnce, so a green dot means a working session
rather than a check made beforehand; while disconnected the bar reads
"Ready" and claims nothing either way.

checkHealth() goes with it. Nothing calls it, and leaving a probe helper
in place invites its reuse.

Pinned in test/session_lifecycle_regression_test.dart so it cannot come
back as a tidy-up. flutter analyze clean, 10/10 tests pass.

Co-Authored-By: Claude Opus 5 <[email protected]>

lib/logic/vpn_connection.dart

@@ -38,9 +38,6 @@ class VpnConnection extends ChangeNotifier {
   bool _adoptedRunningRuntime = false;
   bool get adoptedRunningRuntime => _adoptedRunningRuntime;
 
-  bool _serverReachable = false;
-  bool get serverReachable => _serverReachable;
-
   ConnectionMode _activeMode = ConnectionMode.tun;
   ConnectionMode get activeMode => _activeMode;
 
@@ -73,7 +70,6 @@ class VpnConnection extends ChangeNotifier {
     );
   }
 
-  Timer? _healthTimer;
   StreamSubscription<NetworkSnapshot>? _networkSubscription;
 
   SecureSocksSession? _lastSecureSocks;
@@ -142,11 +138,6 @@ class VpnConnection extends ChangeNotifier {
       },
     );
 
-    unawaited(_checkHealth());
-    _healthTimer = Timer.periodic(const Duration(seconds: 30), (_) {
-      unawaited(_checkHealth());
-    });
-
     try {
       final coreVersion = await _vless.getCoreVersion();
       if (!_disposed) {
@@ -472,14 +463,6 @@ class VpnConnection extends ChangeNotifier {
     notifyListeners();
   }
 
-  Future<void> _checkHealth() async {
-    if (_disposed || _status == VpnStatus.connected) return;
-    final reachable = await HivemindService.checkHealth();
-    if (_disposed || _status == VpnStatus.connected) return;
-    _serverReachable = reachable;
-    notifyListeners();
-  }
-
   @override
   void dispose() {
     _disposed = true;
@@ -487,7 +470,6 @@ class VpnConnection extends ChangeNotifier {
     _suppressNativeConnect = true;
     _userDisconnecting = true;
     HivemindService.cancel();
-    _healthTimer?.cancel();
     _networkSubscription?.cancel();
     // Provider/UI disposal is not a user disconnect command. Keeping teardown
     // in disconnect() prevents lifecycle churn from silently dropping the VPN.