fix: guard release signingConfig behind key.properties existence check

signingConfigs.create() was running unconditionally, casting null
properties to String before the buildTypes guard could take effect.
CI has no key.properties so the cast threw. Now falls back to debug
signing when the keystore file is absent.

android/app/build.gradle.kts

@@ -37,11 +37,13 @@ android {
     }
 
     signingConfigs {
-        create("release") {
+        if (keystorePropertiesFile.exists()) {
-            keyAlias = keystoreProperties["keyAlias"] as String
+            create("release") {
-            keyPassword = keystoreProperties["keyPassword"] as String
+                keyAlias = keystoreProperties["keyAlias"] as String
-            storeFile = file(keystoreProperties["storeFile"] as String)
+                keyPassword = keystoreProperties["keyPassword"] as String
-            storePassword = keystoreProperties["storePassword"] as String
+                storeFile = file(keystoreProperties["storeFile"] as String)
+                storePassword = keystoreProperties["storePassword"] as String
+            }
         }
     }