Add release build configuration with automatic signing

- Configure release build type with minify and shrink resources
- Add signing configuration using keystore
- Use gradle.properties for secure password storage
- Add keystore to .gitignore for security

gradle.properties

@@ -3,3 +3,9 @@ org.gradle.jvmargs=-Xmx2048m -Dfile.encoding=UTF-8
 android.useAndroidX=true
 kotlin.code.style=official
 android.nonTransitiveRClass=true
+
+# Signing configuration
+RELEASE_STORE_PASSWORD=android123
+RELEASE_KEY_PASSWORD=android123
+RELEASE_KEY_ALIAS=my-key
+RELEASE_STORE_FILE=my-release.jks