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).

shared/src/commonMain/kotlin/androidx/compose/material/icons/Icons.kt

@@ -1,12 +0,0 @@
-package androidx.compose.material.icons
-
-object Icons {
-    object Filled
-    val Default: Filled get() = Filled
-
-    object Outlined
-
-    object AutoMirrored {
-        object Filled
-    }
-}