fix(audit): P1 心跳自启动 + refreshToken body + 类型修复

审计修复 Batch 2 (M4-03/M7-04/M11-01):

M4-03: 心跳引擎自动启动
  - chat.rs auto-init 块: engine 创建后立即 start()
  - 通过 engines.get() 获取引用避免 move 后使用

M7-04: refreshToken 发送 body 修复
  - SaaSClient 新增 refreshTokenValue 存储 refresh_token
  - refreshToken() 发送 { refresh_token } body
  - SaaSRefreshResponse 新增 refresh_token 字段
  - login/register 自动存储 refresh_token
  - 添加 getRefreshToken/setRefreshToken 访问器

M11-01: blocking_lock 死锁修复 (已存在)
  - 确认 try_lock + Result 匹配模式已正确

desktop/src-tauri/src/kernel_commands/chat.rs

@@ -163,7 +163,11 @@ pub async fn agent_chat_stream(
                 None,
             );
             engines.insert(request.agent_id.clone(), engine);
-            tracing::info!("[agent_chat_stream] Auto-initialized heartbeat for agent: {}", request.agent_id);
+            // Start the engine after insertion via the stored reference
+            if let Some(e) = engines.get(&request.agent_id) {
+                e.start().await;
+            }
+            tracing::info!("[agent_chat_stream] Auto-initialized and started heartbeat for agent: {}", request.agent_id);
         }
     }