fix(proxy): address code review feedback on response handling

Fixes from PR #1714 code review:

- Extract `read_decoded_body()` and `strip_entity_headers_for_rebuilt_body()`
  in response_processor to properly clean content-encoding/content-length
  headers after decompression
- Reuse `read_decoded_body()` in handlers.rs for Claude transform path,
  ensuring compressed responses are decoded before format conversion
- Make `build_proxy_url_from_config()` public so forwarder can pass proxy
  URL to the hyper raw write path
- Add `has_system_proxy_env()` utility with test coverage
- Add 50ms backoff after accept() failures in server.rs to prevent
  tight-loop CPU spin on transient socket errors

src-tauri/src/proxy/http_client.rs

@@ -280,6 +280,28 @@ fn system_proxy_points_to_loopback() -> bool {
         .any(|value| proxy_points_to_loopback(&value))
 }
 
+/// 检查当前环境是否存在可用的系统代理配置。
+#[allow(dead_code)]
+pub fn has_system_proxy_env() -> bool {
+    const KEYS: [&str; 6] = [
+        "HTTP_PROXY",
+        "http_proxy",
+        "HTTPS_PROXY",
+        "https_proxy",
+        "ALL_PROXY",
+        "all_proxy",
+    ];
+
+    if system_proxy_points_to_loopback() {
+        return false;
+    }
+
+    KEYS.iter()
+        .filter_map(|key| env::var(key).ok())
+        .map(|value| value.trim().to_string())
+        .any(|value| !value.is_empty())
+}
+
 fn proxy_points_to_loopback(value: &str) -> bool {
     fn host_is_loopback(host: &str) -> bool {
         if host.eq_ignore_ascii_case("localhost") {
@@ -337,7 +359,7 @@ pub fn mask_url(url: &str) -> String {
 /// 根据供应商单独代理配置构建代理 URL
 ///
 /// 将 ProviderProxyConfig 转换为代理 URL 字符串
-fn build_proxy_url_from_config(config: &ProviderProxyConfig) -> Option<String> {
+pub fn build_proxy_url_from_config(config: &ProviderProxyConfig) -> Option<String> {
     let proxy_type = config.proxy_type.as_deref().unwrap_or("http");
     let host = config.proxy_host.as_deref()?;
     let port = config.proxy_port?;
@@ -544,4 +566,19 @@ mod tests {
             std::env::remove_var(key);
         }
     }
+
+    #[test]
+    fn test_has_system_proxy_env_ignores_own_loopback_proxy() {
+        let _guard = env_lock().lock().unwrap();
+        set_proxy_port(15721);
+        std::env::remove_var("HTTP_PROXY");
+
+        std::env::set_var("HTTP_PROXY", "http://127.0.0.1:15721");
+        assert!(!has_system_proxy_env());
+
+        std::env::set_var("HTTP_PROXY", "http://127.0.0.1:7890");
+        assert!(has_system_proxy_env());
+
+        std::env::remove_var("HTTP_PROXY");
+    }
 }