- 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/utility/Utils.kt

@@ -75,8 +75,9 @@ object Utils {
       .filter { it.language in supportedLanguages }
       .let { it.ifEmpty { listOf(Locale.US) } }
     Locale.setDefault(compatibleLocales.first())
-    val newConfiguration = Configuration(configuration)
-      newConfiguration.setLocales(LocaleList(*compatibleLocales.toTypedArray()))
+    val newConfiguration = Configuration(configuration).apply {
+      setLocales(LocaleList(*compatibleLocales.toTypedArray()))
+    }
     return context.createConfigurationContext(newConfiguration)
   }