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/build.rs

@@ -0,0 +1,8 @@
+const COMMANDS: &[&str] = &["get_state", "select_images", "set_setting", "next_wallpaper"];
+
+fn main() {
+  tauri_plugin::Builder::new(COMMANDS)
+    .android_path("android")
+    .ios_path("ios")
+    .build();
+}