fix(macos): reliable Local Network permission denial detection and re-check

- Replace broad POSIX error checks (EACCES/EPERM/ENETDOWN) with the
  canonical kDNSServiceErr_PolicyDenied DNS error in the NWBrowser
  state handler, matching the pattern used by Expo, Pulse, Strongbox,
  and WLED. Detect denial in both .failed and .waiting states.

- Add checkLocalNetworkAccess(host:port:) — a read-only NWConnection
  probe (Sequel-Ace pattern) that checks
  NWPath.unsatisfiedReason == .localNetworkDenied without triggering
  a new system prompt. Useful for confirming denial against a specific
  destination before attempting to connect.

- In _onConnect, after the prompt resolves to Denied, confirm with
  checkLocalNetworkAccess against the target host. If confirmed,
  abort the connect attempt and show a non-modal snackbar with an
  'Open System Settings' action that deep-links to
  Privacy_LocalNetwork. Previously the app would proceed to connect,
  fail with PermissionDenied, and surface a redundant in-app modal.

- Drop the now-orphaned _openIosAppSettings helper and
  _iosPlatformChannel constant (the only caller was the removed
  in-app permission dialog).

- Add unit tests for checkLocalNetworkAccess covering outbound
  MethodCall arguments and state parsing for Granted/Denied.

Trace: SRS-300.

apps/chanora_flutter/lib/services/macos_permissions_service.dart

@@ -63,6 +63,8 @@ const String methodTriggerLocalNetworkPrompt = 'triggerLocalNetworkPrompt';
 @visibleForTesting
 const String methodCheckLocalNetwork = 'checkLocalNetwork';
 @visibleForTesting
+const String methodCheckLocalNetworkAccess = 'checkLocalNetworkAccess';
+@visibleForTesting
 const String methodRequestNotifications = 'requestNotifications';
 @visibleForTesting
 const String methodCheckNotifications = 'checkNotifications';
@@ -440,6 +442,37 @@ class MacOSPermissionsService {
     }
   }
 
+  /// Probe whether Local Network access is currently denied for [host]:[port]
+  /// by creating a short-lived NWConnection and checking
+  /// `unsatisfiedReason == .localNetworkDenied`.
+  ///
+  /// This does NOT trigger a new system prompt — it is a read-only check.
+  /// Returns [MacOSLocalNetworkState.unsupported] on non-macOS platforms.
+  Future<MacOSLocalNetworkState> checkLocalNetworkAccess({
+    required String host,
+    required int port,
+  }) async {
+    final ch = _channel;
+    if (ch == null) return MacOSLocalNetworkState.unsupported;
+    try {
+      final raw = await ch.invokeMethod<String>(
+        methodCheckLocalNetworkAccess,
+        <String, dynamic>{'host': host, 'port': port},
+      );
+      final state = _parseLocalNetworkState(raw);
+      _localNetworkState.value = state;
+      return state;
+    } catch (e, st) {
+      developer.log(
+        'checkLocalNetworkAccess failed',
+        name: 'MacOSPermissionsService',
+        error: e,
+        stackTrace: st,
+      );
+      return _localNetworkState.value;
+    }
+  }
+
   // -- Outbound: Notifications ----------------------------------------------
 
   Future<MacOSPermissionState> _checkNotifications() async {