Fix Gradle config crash when key.properties is absent

signingConfigs.release was configured unconditionally, so even assembleDebug
failed at configuration time with 'getProperty(...) must not be null' on CI
(build-apk.yml restores the keystore only after the debug build). Guard the
release signing config on key.properties existence: debug builds proceed
unsigned, release builds still sign when the keystore is present.

android/app/build.gradle.kts

@@ -9,9 +9,10 @@ plugins {
 // Release signing: android/key.properties (gitignored) points to a permanent
 // keystore outside the repo, so every release build has the same signature and
 // installs update in place without wiping app data.
-val keystoreProperties = Properties().apply {
-    val file = rootProject.file("key.properties")
-    if (file.exists()) file.inputStream().use { load(it) }
+val keystorePropertiesFile = rootProject.file("key.properties")
+val keystoreProperties = Properties()
+if (keystorePropertiesFile.exists()) {
+    keystorePropertiesFile.inputStream().use { keystoreProperties.load(it) }
 }
 
 android {
@@ -36,17 +37,24 @@ android {
     }
 
     signingConfigs {
-        create("release") {
-            storeFile = file(keystoreProperties.getProperty("storeFile"))
-            storePassword = keystoreProperties.getProperty("storePassword")
-            keyAlias = keystoreProperties.getProperty("keyAlias")
-            keyPassword = keystoreProperties.getProperty("keyPassword")
+        if (keystorePropertiesFile.exists()) {
+            create("release") {
+                storeFile = file(keystoreProperties.getProperty("storeFile"))
+                storePassword = keystoreProperties.getProperty("storePassword")
+                keyAlias = keystoreProperties.getProperty("keyAlias")
+                keyPassword = keystoreProperties.getProperty("keyPassword")
+            }
         }
     }
 
     buildTypes {
         release {
-            signingConfig = signingConfigs.getByName("release")
+            // No signing config when key.properties is absent (CI debug builds,
+            // local dev) — APK is simply unsigned. With key.properties present,
+            // releases are signed with the permanent keystore.
+            if (keystorePropertiesFile.exists()) {
+                signingConfig = signingConfigs.getByName("release")
+            }
         }
         debug {
             // Install debug builds under a separate applicationId so they don't