Add release build config and Play Store prep

- Add signing config (reads from env vars)
- Enable R8 minification and resource shrinking
- Expand ProGuard rules for Gson
- Update README with build instructions and Play Store checklist

app/build.gradle.kts

@@ -15,13 +15,27 @@ android {
         versionName = "1.0.0"
     }
 
+    signingConfigs {
+        create("release") {
+            val keystoreFile = file("../keystore.jks")
+            if (keystoreFile.exists()) {
+                storeFile = keystoreFile
+                storePassword = System.getenv("KEYSTORE_PASSWORD") ?: ""
+                keyAlias = System.getenv("KEY_ALIAS") ?: "release"
+                keyPassword = System.getenv("KEY_PASSWORD") ?: ""
+            }
+        }
+    }
+
     buildTypes {
         release {
-            isMinifyEnabled = false
+            isMinifyEnabled = true
+            isShrinkResources = true
             proguardFiles(
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"
             )
+            signingConfig = signingConfigs.findByName("release")
         }
     }