refactor(proxy): distinguish circuit-open from no-provider errors

src-tauri/src/error.rs

@@ -52,6 +52,10 @@ pub enum AppError {
     },
     #[error("数据库错误: {0}")]
     Database(String),
+    #[error("所有供应商已熔断，无可用渠道")]
+    AllProvidersCircuitOpen,
+    #[error("未配置供应商")]
+    NoProvidersConfigured,
 }
 
 impl AppError {