feat(setup): bootstrap Android plugin and web UI

This initial commit adds a multi-part scaffold for Wechselbild, including a web UI and an Android plugin.
It introduces frontend assets and a package.json-based dev setup, plus plugin scaffolding for Android with Kotlin sources and Cargo.toml for the Tauri plugin.

- Adds Android wallpaper plugin with Kotlin sources and manifest
- Includes web UI scaffold with index.html and package.json
- Adds Tauri plugin scaffold with Cargo.toml and build config

src-tauri/gen/android/app/.gitignore

@@ -0,0 +1,6 @@
+/src/main/**/generated
+/src/main/jniLibs/**/*.so
+/src/main/assets/tauri.conf.json
+/tauri.build.gradle.kts
+/proguard-tauri.pro
+/tauri.properties