feat: configurer la signature release Android

Le build release lit androidApp/keystore.properties (gitigné) pour signer
l'AAB ; sans ce fichier, il échoue volontairement. La keystore servira de
clé d'upload Play : la section Publication du README documente la
procédure et la nécessité de la sauvegarder.

androidApp/build.gradle.kts

@@ -1,10 +1,16 @@
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import java.util.Properties
 
 plugins {
     alias(libs.plugins.compose.compiler)
     alias(libs.plugins.android.application)
 }
 
+val keystoreProperties = Properties().apply {
+    val f = file("keystore.properties")
+    if (f.exists()) f.inputStream().use { load(it) }
+}
+
 android {
     namespace = "com.livrets.androidApp"
     compileSdk = 37
@@ -18,6 +24,24 @@ android {
         versionName = "1.0.0"
     }
 
+    signingConfigs {
+        create("release") {
+            if (keystoreProperties.isNotEmpty()) {
+                storeFile = file(keystoreProperties["storeFile"] as String)
+                storePassword = keystoreProperties["storePassword"] as String
+                keyAlias = keystoreProperties["keyAlias"] as String
+                keyPassword = keystoreProperties["keyPassword"] as String
+            }
+        }
+    }
+
+    buildTypes {
+        release {
+            isMinifyEnabled = false
+            signingConfig = signingConfigs.getByName("release")
+        }
+    }
+
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_17
         targetCompatibility = JavaVersion.VERSION_17