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/commands.rs

@@ -0,0 +1,10 @@
+use tauri::{AppHandle, command, Runtime};
+
+use crate::models::*;
+use crate::Result;
+use crate::WallpaperExt;
+
+#[command] pub(crate) async fn get_state<R: Runtime>(app: AppHandle<R>) -> Result<WallpaperState> { app.wallpaper().get_state() }
+#[command] pub(crate) async fn select_images<R: Runtime>(app: AppHandle<R>) -> Result<WallpaperState> { app.wallpaper().select_images() }
+#[command] pub(crate) async fn set_setting<R: Runtime>(app: AppHandle<R>, name: String, value: bool) -> Result<WallpaperState> { app.wallpaper().set_setting(SettingRequest { name, value }) }
+#[command] pub(crate) async fn next_wallpaper<R: Runtime>(app: AppHandle<R>) -> Result<WallpaperState> { app.wallpaper().next_wallpaper() }