ci: add optional release signing from repository secrets

app/build.gradle.kts

@@ -1,9 +1,23 @@
+import java.util.Properties
+
 plugins {
     id("com.android.application")
     id("org.jetbrains.kotlin.android")
     id("org.jetbrains.kotlin.plugin.compose")
 }
 
+val keystorePropertiesFile = rootProject.file("keystore.properties")
+val keystoreProperties = Properties()
+if (keystorePropertiesFile.exists()) {
+    keystorePropertiesFile.inputStream().use { keystoreProperties.load(it) }
+}
+val hasReleaseSigning = listOf(
+    "storeFile",
+    "storePassword",
+    "keyAlias",
+    "keyPassword",
+).all { !keystoreProperties.getProperty(it).isNullOrBlank() }
+
 android {
     namespace = "solutions.tretter.esunandroid"
     compileSdk = 35
@@ -19,6 +33,17 @@ android {
         vectorDrawables.useSupportLibrary = true
     }
 
+    signingConfigs {
+        if (hasReleaseSigning) {
+            create("release") {
+                storeFile = file(keystoreProperties.getProperty("storeFile"))
+                storePassword = keystoreProperties.getProperty("storePassword")
+                keyAlias = keystoreProperties.getProperty("keyAlias")
+                keyPassword = keystoreProperties.getProperty("keyPassword")
+            }
+        }
+    }
+
     buildTypes {
         release {
             isMinifyEnabled = false
@@ -26,6 +51,9 @@ android {
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"
             )
+            if (hasReleaseSigning) {
+                signingConfig = signingConfigs.getByName("release")
+            }
         }
     }