fix: warn about insecure custom API server connections (#16616)

* fix: warn about insecure custom API server connections

Signed-off-by: fufesou <linlong1266@gmail.com>

* fix: limit API server warning to HTTP addresses

Signed-off-by: fufesou <linlong1266@gmail.com>

* fix: resolve API warnings against effective server configuration

Signed-off-by: fufesou <linlong1266@gmail.com>

* fix: reuse common sync for API server previews

Signed-off-by: fufesou <linlong1266@gmail.com>

* fix: keep server settings usable on API preview errors

Signed-off-by: fufesou <linlong1266@gmail.com>

---------

Signed-off-by: fufesou <linlong1266@gmail.com>

flutter/lib/common/widgets/api_server_warning.dart

@@ -0,0 +1,72 @@
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_hbb/models/platform_model.dart';
+
+String _resolveApiServer({required String api, required String custom}) {
+  final key = 'resolve-api-server:${jsonEncode([api, custom])}';
+  final response = jsonDecode(bind.mainGetCommonSync(key: key));
+  if (response['error'] != null) {
+    throw FormatException('Failed to resolve API server: ${response['error']}');
+  }
+  return response['url'] as String;
+}
+
+bool shouldWarnApiServer(String? apiServer) {
+  final uri = Uri.tryParse(apiServer?.trim() ?? '');
+  if (uri == null || uri.host.isEmpty || uri.scheme != 'http') {
+    return false;
+  }
+  return !const {'localhost', '127.0.0.1', '::1'}.contains(uri.host);
+}
+
+class ApiServerWarning extends StatelessWidget {
+  final TextEditingController controller;
+  final TextEditingController idController;
+  final String message;
+  final String emptyMessage;
+  final String Function({required String api, required String custom})
+      resolveApiServer;
+
+  const ApiServerWarning({
+    super.key,
+    required this.controller,
+    required this.idController,
+    required this.message,
+    required this.emptyMessage,
+    this.resolveApiServer = _resolveApiServer,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return ListenableBuilder(
+      listenable: Listenable.merge([controller, idController]),
+      builder: (context, child) {
+        final apiServer = controller.text;
+        final idServer = idController.text;
+        final String effectiveApiServer;
+        try {
+          effectiveApiServer =
+              resolveApiServer(api: apiServer.trim(), custom: idServer.trim());
+        } catch (error, stack) {
+          FlutterError.reportError(FlutterErrorDetails(
+            exception: error,
+            stack: stack,
+            context: ErrorDescription('while resolving the API server warning'),
+          ));
+          return const SizedBox.shrink();
+        }
+        if (!shouldWarnApiServer(effectiveApiServer)) {
+          return const SizedBox.shrink();
+        }
+        return Align(
+          alignment: AlignmentDirectional.centerStart,
+          child: Text(
+            controller.text.trim().isEmpty ? emptyMessage : message,
+            style: TextStyle(color: Colors.orange.shade700),
+          ),
+        );
+      },
+    );
+  }
+}