feat(backend): Phase 2.1 scaffolding for bookshelf-api + ABS

- Port BookshelfApiService, AudiobookshelfApiService and all models from Book's Story.
- Add Hilt 2.56.1 plugin, @HiltAndroidApp, NetworkModule, BackendModule.
- Add Retrofit/OkHttp/Kotlinx Serialization dependencies.
- Provide minimal SharedPreferences-based ServerSettings for URL/token storage.
- Copy UriUtils (fixUriScheme/normalizeUri) from Book's Story.
- No wiring into UI yet; OPDS/Gutenberg/LocalFolder still present.
- ':app:assembleOssDebug' passes (APK ~80 MB).

build.gradle.kts

@@ -6,6 +6,7 @@ plugins {
     alias(libs.plugins.kotlin.compose) apply false
     alias(libs.plugins.kotlin.serialization) apply false
     alias(libs.plugins.kotlin.ksp) apply false
+    alias(libs.plugins.hilt) apply false
     alias(libs.plugins.kover) apply false
 }