build: add release signing config and Zapstore publish config

- release signing reads keystore/keystore.properties (git-ignored)
- zapstore.yaml publishes the app via local APK source

app/build.gradle.kts

@@ -3,6 +3,14 @@ plugins {
     id("org.jetbrains.kotlin.android")
 }
 
+import java.util.Properties
+
+val keystoreProperties = Properties()
+val keystorePropertiesFile = rootProject.file("keystore/keystore.properties")
+if (keystorePropertiesFile.exists()) {
+    keystoreProperties.load(keystorePropertiesFile.inputStream())
+}
+
 android {
     namespace = "lostmyphone.app"
     compileSdk = 34
@@ -15,9 +23,21 @@ android {
         versionName = "1.0"
     }
 
+    signingConfigs {
+        if (keystorePropertiesFile.exists()) {
+            create("release") {
+                storeFile = rootProject.file(keystoreProperties.getProperty("storeFile"))
+                storePassword = keystoreProperties.getProperty("storePassword")
+                keyAlias = keystoreProperties.getProperty("keyAlias")
+                keyPassword = keystoreProperties.getProperty("keyPassword")
+            }
+        }
+    }
+
     buildTypes {
         release {
             isMinifyEnabled = false
+            signingConfig = signingConfigs.findByName("release")
             proguardFiles(
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"