feat: improve endpoint speed test UI and accuracy

- Add color-coded latency indicators (green <300ms, yellow <500ms, orange <800ms, red >=800ms)
- Fix speed test button width to prevent layout shift during testing
- Clear latency data before each test to properly show loading state
- Add warmup request in speed test to avoid first packet penalty and improve accuracy

src-tauri/src/speedtest.rs

@@ -65,6 +65,10 @@ pub async fn test_endpoints(
                 }
             };
 
+            // 先进行一次“热身”请求，忽略其结果，仅用于复用连接/绕过首包惩罚
+            let _ = client.get(parsed_url.clone()).send().await;
+
+            // 第二次请求开始计时，并将其作为结果返回
             let start = Instant::now();
             match client.get(parsed_url).send().await {
                 Ok(resp) => {