Add ShadowJar plugin for shaded build

Configure package relocation to avoid dependency conflicts.
Update build process to produce minimized ShadowJar artifacts and disable default jar task.

build.gradle.kts

@@ -1,11 +1,22 @@
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
+
 plugins {
     id("java")
+    id("com.gradleup.shadow") version "9.0.0-beta17"
 }
 
 group = "com.alttd.webinterface"
 version = "1.0-SNAPSHOT"
 description = "WebInterface"
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(21))
+    }
+}
+
 repositories {
     mavenCentral()
     maven("https://nexus.velocitypowered.com/repository/maven-public/")
@@ -25,3 +36,25 @@ dependencies {
 tasks.test {
     useJUnitPlatform()
 }
+
+tasks.withType<ShadowJar> {
+    // Relocate packages to avoid conflicts with other plugins
+    relocate("io.javalin", "com.alttd.webinterface.shaded.io.javalin")
+    relocate("org.slf4j", "com.alttd.webinterface.shaded.org.slf4j")
+    relocate("kotlin", "com.alttd.webinterface.shaded.kotlin")
+    relocate("org.eclipse.jetty", "com.alttd.webinterface.shaded.org.eclipse.jetty")
+
+    minimize()
+}
+
+tasks.build {
+    dependsOn(tasks.shadowJar)
+}
+
+tasks.jar {
+    enabled = false
+}
+
+artifacts {
+    archives(tasks.shadowJar)
+}