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/mobile.rs

@@ -0,0 +1,32 @@
+use serde::de::DeserializeOwned;
+use tauri::{
+  plugin::{PluginApi, PluginHandle},
+  AppHandle, Runtime,
+};
+
+use crate::models::*;
+
+#[cfg(target_os = "ios")]
+tauri::ios_plugin_binding!(init_plugin_wallpaper);
+
+// initializes the Kotlin or Swift plugin classes
+pub fn init<R: Runtime, C: DeserializeOwned>(
+  _app: &AppHandle<R>,
+  api: PluginApi<R, C>,
+) -> crate::Result<Wallpaper<R>> {
+  #[cfg(target_os = "android")]
+  let handle = api.register_android_plugin("de.wechselbild.wallpaper", "WallpaperPlugin")?;
+  #[cfg(target_os = "ios")]
+  let handle = api.register_ios_plugin(init_plugin_wallpaper)?;
+  Ok(Wallpaper(handle))
+}
+
+/// Access to the wallpaper APIs.
+pub struct Wallpaper<R: Runtime>(PluginHandle<R>);
+
+impl<R: Runtime> Wallpaper<R> {
+  pub fn get_state(&self) -> crate::Result<WallpaperState> { self.0.run_mobile_plugin("getState", ()).map_err(Into::into) }
+  pub fn select_images(&self) -> crate::Result<WallpaperState> { self.0.run_mobile_plugin("selectImages", ()).map_err(Into::into) }
+  pub fn set_setting(&self, payload: SettingRequest) -> crate::Result<WallpaperState> { self.0.run_mobile_plugin("setSetting", payload).map_err(Into::into) }
+  pub fn next_wallpaper(&self) -> crate::Result<WallpaperState> { self.0.run_mobile_plugin("nextWallpaper", ()).map_err(Into::into) }
+}