feat: embed a Flutter G4F client with Cronet, cookie jar, and origin WebView

Port the Oracle off the Python g4f container onto on-device providers: Chromium HTTP via Cronet, a shared cookie session, IterList retry, hosted OpenAI-compat aggregators, and same-origin WebView fallbacks instead of CORS fetch.

lib/g4f/models.dart

@@ -0,0 +1,26 @@
+// models.dart
+enum Role { system, user, assistant }
+
+class Message {
+  final Role role;
+  final String content;
+
+  const Message({required this.role, required this.content});
+
+  Map<String, dynamic> toJson() => {
+        'role': role.name,
+        'content': content,
+      };
+}
+
+class G4FModels {
+  static const String gpt4 = 'gpt-4';
+  static const String gpt4o = 'gpt-4o';
+  static const String gpt4oMini = 'gpt-4o-mini';
+  static const String commandR = 'command-r';
+  static const String commandRPlus = 'command-r-plus';
+  static const String mistral = 'mistral';
+  static const String llama3 = 'llama-3';
+  static const String qwen = 'qwen';
+  static const String openaiFast = 'openai-fast';
+}