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/lib/main.dart

@@ -648,11 +648,18 @@ class _BetaHomeState extends State<_BetaHome> with WidgetsBindingObserver {
         action: SnackBarAction(
           label: l10n.networkPermissionOpenSettings,
           onPressed: () {
-            try {
-              Process.run('open', [
-                'x-apple.systempreferences:com.apple.preference.security?Privacy_LocalNetwork',
-              ]);
-            } catch (_) {}
+            // Fire-and-forget: don't block snackbar dismissal on the
+            // child process. `Process.run` may throw synchronously
+            // (e.g. exec ENOENT) or return a future that rejects
+            // (e.g. macOS denies fork); both paths are swallowed
+            // because the user can still open Settings manually.
+            unawaited(
+              Future<void>(() async {
+                await Process.run('open', [
+                  'x-apple.systempreferences:com.apple.preference.security?Privacy_LocalNetwork',
+                ]);
+              }).catchError((_) {}),
+            );
           },
         ),
       ),