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/components/status_bar.dart

@@ -49,15 +49,14 @@ class StatusBar extends StatelessWidget {
       builder: (context, vpn, timer, _) {
         final isConnected = vpn.status == VpnStatus.connected;
         final isConnecting = vpn.status == VpnStatus.connecting;
-        final serverHealthy =
-            vpn.serverReachable || (isConnected && timer.hasSyncedOnce);
-        final sessionOrTunnelHealthy = timer.hasSyncedOnce || vpn.serverReachable;
+        // Rule 60: reachability is known only from a session sync that
+        // succeeded, never from a probe. While disconnected the app knows
+        // nothing about the server, so it claims nothing either way.
+        final serverHealthy = isConnected && timer.hasSyncedOnce;
 
         final statusLabel = isConnected
-            ? (sessionOrTunnelHealthy ? 'Online' : 'Syncing…')
-            : (isConnecting
-                ? 'Connecting…'
-                : (vpn.serverReachable ? 'Ready' : 'Offline'));
+            ? (timer.hasSyncedOnce ? 'Online' : 'Syncing…')
+            : (isConnecting ? 'Connecting…' : 'Ready');
 
         final detail = _detail(vpn, timer);