fix(macos): address PR #31 review findings

- _showLocalNetworkDeniedSnackBar: wrap Process.run with unawaited()
  and .catchError() so a rejected future (e.g. macOS sandbox refuses
  fork, or 'open' is missing) cannot bubble into the Flutter zone
  as an unhandled exception. The synchronous try/catch was a no-op
  because Process.run only throws asynchronously.
- macos_permissions_service_test.dart: mirror the
  triggerLocalNetworkPrompt error-handling test with one for
  checkLocalNetworkAccess. Probe-path failures (NWConnection probe
  cannot establish, or Swift side throws) must fall back to cached
  state without crashing the caller.

Tests: 186 passed, 2 skipped. Dart analyze clean.

apps/chanora_flutter/test/services/macos_permissions_service_test.dart

@@ -557,4 +557,30 @@ void main() {
       svc.dispose();
     },
   );
+
+  test(
+    'SWE4-UV / SRS-300: checkLocalNetworkAccess() returns cached state '
+    'when the channel throws',
+    () async {
+      final svc = MacOSPermissionsService(channel: channel)..start();
+
+      outgoingResponder = (call) async {
+        if (call.method == methodCheckLocalNetworkAccess) {
+          throw PlatformException(code: 'probe-failed');
+        }
+        return null;
+      };
+
+      // Probe path failures (e.g. NWConnection couldn't establish a
+      // listener, or the Swift side threw) must not crash callers
+      // — they must fall back to whatever the service already cached.
+      final result = await svc.checkLocalNetworkAccess(
+        host: '127.0.0.1',
+        port: 9987,
+      );
+
+      expect(result, isNotNull);
+      svc.dispose();
+    },
+  );
 }