fix: add signing config so release APK can be installed

The release build had no signing config, producing an unsigned APK
that Android refuses to install. Added debug keystore signing for
release builds and a CI step to ensure the keystore exists.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -11,6 +11,15 @@ android {
     namespace = "pl.firmatpp.itstransport"
     compileSdk = 35
 
+    signingConfigs {
+        create("release") {
+            storeFile = file("${System.getProperty("user.home")}/.android/debug.keystore")
+            storePassword = "android"
+            keyAlias = "androiddebugkey"
+            keyPassword = "android"
+        }
+    }
+
     defaultConfig {
         applicationId = "pl.firmatpp.itstransport"
         minSdk = 26
@@ -36,6 +45,7 @@ android {
             )
         }
         release {
+            signingConfig = signingConfigs.getByName("release")
             isMinifyEnabled = false
             proguardFiles(
                 getDefaultProguardFile("proguard-android-optimize.txt"),