feat(proxy): add modify proxy API endpoint and functionality

Implement proxy modification feature including:
- New modifyFrpcProxy function in config.go
- New ModifyProxyHandler in frpcProxyAct.go
- New API endpoint in router.go
- Updated API documentation in docs/api.md

config.go

@@ -294,6 +294,39 @@ func removeFrpcProxy(configContent string, proxyName string) (string, error) {
 	return result, nil
 }
 
+func modifyFrpcProxy(configContent string, info FrpcProxyInfo) (string, error) {
+	config, err := decodeFrpcConfig(configContent)
+	if err != nil {
+		return "", fmt.Errorf("failed to parse config: %w", err)
+	}
+
+	var found bool
+	for i, proxy := range config.Proxies {
+		if name, ok := proxy["name"].(string); ok && name == info.Name {
+			config.Proxies[i] = map[string]interface{}{
+				"name":       info.Name,
+				"type":       info.Type,
+				"localIP":    info.LocalIP,
+				"localPort":  info.LocalPort,
+				"remotePort": info.RemotePort,
+			}
+			found = true
+			break
+		}
+	}
+
+	if !found {
+		return "", fmt.Errorf("proxy %s not found", info.Name)
+	}
+
+	result, err := encodeFrpcConfig(config)
+	if err != nil {
+		return "", fmt.Errorf("failed to write config: %w", err)
+	}
+
+	return result, nil
+}
+
 func getKeyText(configPath, key, section string) (value string, err error) {
 	configContent, err := os.ReadFile(configPath)
 	if err != nil {