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/schemas/capabilities.json

@@ -0,0 +1 @@
+{"default":{"identifier":"default","description":"Standardrechte für Wechselbild","local":true,"windows":["main"],"permissions":["core:default","wallpaper:default"]}}