feat(build): KMP → Android-only migration (Phase 1.2)

- Delete desktopApp/ and shared/ modules.
- Merge shared commonMain + androidMain + readerJvmMain sources into app/src/main/kotlin.
- Resolve expect/actual declarations by keeping android implementations.
- Remove project(':shared') dependency and KMP plugins from build files.
- Simplify settings.gradle.kts to single :app module; root project renamed BookReader.
- Remove JVM-only code (javax.imageio/ImageIO in SharedJvmBookLoader).
- Make LocalFolderSync helpers public for cross-package use in Android module.
- Baseline ':app:assembleOssDebug' passes (APK ~80 MB).

app/build.gradle.kts

@@ -217,8 +217,6 @@ kover {
 //noinspection UseTomlInstead
 dependencies {
 
-    implementation(project(":shared"))
-
     implementation(libs.androidx.core.ktx)
     implementation(libs.androidx.lifecycle.runtime.ktx)
     implementation(libs.androidx.activity.compose)