style: move checkstyle to config/checkstyle and add regex header check

Author: WhatDamon

build.gradle.kts

@@ -93,9 +93,10 @@ tasks.test {
  }
 
 checkstyle {
- 	toolVersion = "10.17.0"
- 	configFile = file("checkstyle.xml")
- }
+  	toolVersion = "10.17.0"
+  	configFile = file("config/checkstyle/checkstyle.xml")
+  	configProperties["checkstyle.config.location"] = "${project.rootDir}/config/checkstyle"
+  }
 
 tasks.checkstyleMain {
  	reports {

checkstyle.xml config/checkstyle/checkstyle.xmlcheckstyle.xml renamed to config/checkstyle/checkstyle.xml

@@ -6,6 +6,10 @@
 <module name="Checker">
     <property name="severity" value="warning"/>
     <property name="fileExtensions" value="java"/>
+    
+    <module name="RegexpHeader">
+        <property name="headerFile" value="${checkstyle.config.location}/license_header.txt"/>
+    </module>
 
     <module name="TreeWalker">
         <module name="NeedBraces">

config/checkstyle/license_header.txt

@@ -0,0 +1,15 @@
+^/\*$
+^ \* Copyright \d{4}(-\d{4})? +Damon +Lu and open-source contributors$
+^ \*$
+^ \* Licensed under the Apache License, Version 2\.0 \(the \"License\"\)\;$
+^ \* you may not use this file except in compliance with the License.$
+^ \* You may obtain a copy of the License at$
+^ \*$
+^ \*     http://www\.apache\.org/licenses/LICENSE\-2\.0$
+^ \*$
+^ \* Unless required by applicable law or agreed to in writing, software$
+^ \* distributed under the License is distributed on an "AS IS" BASIS,$
+^ \* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied\.$
+^ \* See the License for the specific language governing permissions and$
+^ \* limitations under the License\.$
+^ \*/$

src/main/java/org/damon233/performtrackermod/network/UdpMetricsClient.java

@@ -1,3 +1,19 @@
+/*
+ * Copyright 2026 Damon Lu and open-source contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 package org.damon233.performtrackermod.network;
 
 import org.damon233.performtrackermod.PerformTracker;