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

plugins/src/lib.rs

@@ -0,0 +1,48 @@
+use tauri::{
+  plugin::{Builder, TauriPlugin},
+  Manager, Runtime,
+};
+
+pub use models::*;
+
+#[cfg(desktop)]
+mod desktop;
+#[cfg(mobile)]
+mod mobile;
+
+mod commands;
+mod error;
+mod models;
+
+pub use error::{Error, Result};
+
+#[cfg(desktop)]
+use desktop::Wallpaper;
+#[cfg(mobile)]
+use mobile::Wallpaper;
+
+/// Extensions to [`tauri::App`], [`tauri::AppHandle`] and [`tauri::Window`] to access the wallpaper APIs.
+pub trait WallpaperExt<R: Runtime> {
+  fn wallpaper(&self) -> &Wallpaper<R>;
+}
+
+impl<R: Runtime, T: Manager<R>> crate::WallpaperExt<R> for T {
+  fn wallpaper(&self) -> &Wallpaper<R> {
+    self.state::<Wallpaper<R>>().inner()
+  }
+}
+
+/// Initializes the plugin.
+pub fn init<R: Runtime>() -> TauriPlugin<R> {
+  Builder::new("wallpaper")
+    .invoke_handler(tauri::generate_handler![commands::get_state, commands::select_images, commands::set_setting, commands::next_wallpaper])
+    .setup(|app, api| {
+      #[cfg(mobile)]
+      let wallpaper = mobile::init(app, api)?;
+      #[cfg(desktop)]
+      let wallpaper = desktop::init(app, api)?;
+      app.manage(wallpaper);
+      Ok(())
+    })
+    .build()
+}