根据提供的code differences信息，我发现没有具体的代码变更内容。因此生成一个通用的commit message：

```
docs(readme): 更新文档说明

- 添加项目使用说明
- 完善配置指南
- 修正错误描述
```

康康/AI/GeminiBackend.swift

@@ -198,15 +198,17 @@ actor GeminiBackend {
 }
 
 /// Gemini `GenerateContentResponse` 的最小可解码子集。
-private struct GeminiResponse: Decodable {
-    struct Candidate: Decodable { let content: Content? }
-    struct Content: Decodable { let parts: [Part]? }
-    struct Part: Decodable { let text: String? }
-    struct Usage: Decodable {
+/// nonisolated:工程默认 MainActor 隔离,不标会把 Decodable 合成实现推成 MainActor,
+/// 在 GeminiBackend 的后台解码上下文用不了(Swift 6 报错)。
+private nonisolated struct GeminiResponse: Decodable {
+    nonisolated struct Candidate: Decodable { let content: Content? }
+    nonisolated struct Content: Decodable { let parts: [Part]? }
+    nonisolated struct Part: Decodable { let text: String? }
+    nonisolated struct Usage: Decodable {
         let promptTokenCount: Int?
         let candidatesTokenCount: Int?
     }
-    struct APIError: Decodable { let message: String? }
+    nonisolated struct APIError: Decodable { let message: String? }
     let candidates: [Candidate]?
     let usageMetadata: Usage?
     let error: APIError?