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/hivemind_service.dart

@@ -197,18 +197,6 @@ class HivemindService {
     throw Exception('Session not activated. Server callback may have timed out.');
   }
 
-  static Future<bool> checkHealth() async {
-    try {
-      final response = await controlGet(
-        _publicUrl('/health'),
-        timeout: const Duration(seconds: 3),
-      );
-      return response.statusCode == 200;
-    } catch (_) {
-      return false;
-    }
-  }
-
   static String newNonce() {
     final random = Random.secure();
     final high = random.nextInt(0x7FFFFFFF);