Add release signing config for Google Play upload

Read signing credentials from keystore.properties (gitignored) and
configure the release build type to use the upload keystore.

.gitignore

@@ -10,3 +10,5 @@
 .externalNativeBuild
 .cxx
 local.properties
+keystore.properties
+*.jks

app/build.gradle.kts

@@ -7,10 +7,27 @@ plugins {
     alias(libs.plugins.ksp)
 }
 
+import java.util.Properties
+
+val keystorePropertiesFile = rootProject.file("keystore.properties")
+val keystoreProperties = Properties()
+if (keystorePropertiesFile.exists()) {
+    keystoreProperties.load(keystorePropertiesFile.inputStream())
+}
+
 android {
     namespace = "cz.bugsy.karemote"
     compileSdk = 36
 
+    signingConfigs {
+        create("release") {
+            storeFile = rootProject.file(keystoreProperties["storeFile"] as String)
+            storePassword = keystoreProperties["storePassword"] as String?
+            keyAlias = keystoreProperties["keyAlias"] as String?
+            keyPassword = keystoreProperties["keyPassword"] as String?
+        }
+    }
+
     defaultConfig {
         applicationId = "cz.bugsy.karemote"
         minSdk = 33
@@ -31,6 +48,7 @@ android {
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"
             )
+            signingConfig = signingConfigs.getByName("release")
         }
     }
     compileOptions {