- Migrate build system to Kotlin DSL (build.gradle.kts, settings.gradle.kts)

- Upgrade `compileSdk` and `targetSdk` to 36
- Add German (`de`) localization and string resources
- Add Michachatz F-Droid repository to default list
- Update dependencies and plugins in `libs.versions.toml`
- Remove manual signing configuration from README.md
- Refactor code to use non-deprecated `alpha` property and context-based string retrieval
- Clean up `buildConfig` and locale filtering logic

src/main/kotlin/com/michatec/store/graphics/DrawableWrapper.kt

@@ -39,7 +39,7 @@ open class DrawableWrapper(val drawable: Drawable): Drawable() {
   }
 
   override fun getAlpha(): Int {
-    return DrawableCompat.getAlpha(drawable)
+    return drawable.alpha
   }
 
   override fun setAlpha(alpha: Int) {