SAF初步修改，等待验证

lib/models/saf_permission_state.dart

@@ -0,0 +1,63 @@
+/// SAF 权限状态
+enum SafPermissionState {
+  /// 未授权（首次使用）
+  unauthorized,
+
+  /// 已授权，Uri 有效
+  authorized,
+
+  /// 授权已过期或 Uri 失效
+  expired,
+
+  /// 用户拒绝了授权
+  denied,
+}
+
+/// SAF 授权结果
+class SafAuthorizationResult {
+  final bool success;
+  final String? uriString;
+  final SafPermissionState state;
+  final String? errorMessage;
+
+  SafAuthorizationResult(
+    {
+    required this.success,
+    this.uriString,
+    required this.state,
+    this.errorMessage,
+  }
+  );
+
+  factory SafAuthorizationResult.success(String uri) {
+    return SafAuthorizationResult(
+      success: true,
+      uriString: uri,
+      state: SafPermissionState.authorized,
+    );
+  }
+
+  factory SafAuthorizationResult.denied() {
+    return SafAuthorizationResult(
+      success: false,
+      state: SafPermissionState.denied,
+      errorMessage: '用户拒绝了文件夹访问权限',
+    );
+  }
+
+  factory SafAuthorizationResult.expired() {
+    return SafAuthorizationResult(
+      success: false,
+      state: SafPermissionState.expired,
+      errorMessage: '文件夹访问权限已过期，请重新授权',
+    );
+  }
+
+  factory SafAuthorizationResult.error(String message) {
+    return SafAuthorizationResult(
+      success: false,
+      state: SafPermissionState.unauthorized,
+      errorMessage: message,
+    );
+  }
+}