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/desktop.rs

@@ -0,0 +1,22 @@
+use serde::de::DeserializeOwned;
+use tauri::{plugin::PluginApi, AppHandle, Runtime};
+
+use crate::models::*;
+
+pub fn init<R: Runtime, C: DeserializeOwned>(
+  app: &AppHandle<R>,
+  _api: PluginApi<R, C>,
+) -> crate::Result<Wallpaper<R>> {
+  Ok(Wallpaper(app.clone()))
+}
+
+/// Access to the wallpaper APIs.
+pub struct Wallpaper<R: Runtime>(AppHandle<R>);
+
+impl<R: Runtime> Wallpaper<R> {
+  fn demo() -> WallpaperState { WallpaperState { image_count: 3, enabled: true, shuffle: true, lock_screen_only: true, current_index: 0, image_urls: vec!["/wallpapers/alpine.png".into(), "/wallpapers/waterfall.png".into(), "/wallpapers/coast.png".into()] } }
+  pub fn get_state(&self) -> crate::Result<WallpaperState> { Ok(Self::demo()) }
+  pub fn select_images(&self) -> crate::Result<WallpaperState> { Ok(Self::demo()) }
+  pub fn set_setting(&self, payload: SettingRequest) -> crate::Result<WallpaperState> { let mut state = Self::demo(); match payload.name.as_str() { "enabled" => state.enabled = payload.value, "shuffle" => state.shuffle = payload.value, "lockScreenOnly" => state.lock_screen_only = payload.value, _ => {} }; Ok(state) }
+  pub fn next_wallpaper(&self) -> crate::Result<WallpaperState> { let mut state = Self::demo(); state.current_index = 1; Ok(state) }
+}