build: load release signing config from local keystore.properties

Conditionally applies the BG7NTA signingConfig when keystore.properties exists (gitignored). CI signs via apksigner with GitHub Secrets, so this only affects local builds.

app/build.gradle.kts

@@ -1,3 +1,29 @@
+import java.util.Properties
+
 plugins {
     alias(libs.plugins.convention.applicationPlugin)
 }
+
+// Load signing config from keystore.properties (gitignored, never commit credentials)
+val keystoreProperties = Properties().apply {
+    val propsFile = rootProject.file("keystore.properties")
+    if (propsFile.exists()) propsFile.inputStream().use { load(it) }
+}
+
+android {
+    signingConfigs {
+        if (keystoreProperties["storeFile"] != null) {
+            create("release") {
+                storeFile = rootProject.file(keystoreProperties["storeFile"] as String)
+                storePassword = keystoreProperties["storePassword"] as String
+                keyAlias = keystoreProperties["keyAlias"] as String
+                keyPassword = keystoreProperties["keyPassword"] as String
+            }
+        }
+    }
+    buildTypes {
+        release {
+            signingConfig = signingConfigs.findByName("release")
+        }
+    }
+}