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

src/native.ts

@@ -0,0 +1,43 @@
+import { invoke } from "@tauri-apps/api/core";
+
+export type WallpaperState = {
+  imageCount: number;
+  enabled: boolean;
+  shuffle: boolean;
+  lockScreenOnly: boolean;
+  currentIndex: number;
+  imageUrls: string[];
+};
+
+const demoState: WallpaperState = {
+  imageCount: 3,
+  enabled: true,
+  shuffle: true,
+  lockScreenOnly: true,
+  currentIndex: 0,
+  imageUrls: ["/wallpapers/alpine.png", "/wallpapers/waterfall.png", "/wallpapers/coast.png"],
+};
+
+const inTauri = () => "__TAURI_INTERNALS__" in window;
+
+export async function getState(): Promise<WallpaperState> {
+  return inTauri() ? invoke<WallpaperState>("plugin:wallpaper|get_state") : demoState;
+}
+
+export async function selectImages(): Promise<WallpaperState> {
+  if (!inTauri()) return demoState;
+  return invoke<WallpaperState>("plugin:wallpaper|select_images");
+}
+
+export async function setSetting(name: "enabled" | "shuffle" | "lockScreenOnly", value: boolean) {
+  if (!inTauri()) return { ...demoState, [name]: value };
+  return invoke<WallpaperState>("plugin:wallpaper|set_setting", { name, value });
+}
+
+export async function nextWallpaper(): Promise<WallpaperState> {
+  if (!inTauri()) {
+    demoState.currentIndex = (demoState.currentIndex + 1) % demoState.imageUrls.length;
+    return { ...demoState };
+  }
+  return invoke<WallpaperState>("plugin:wallpaper|next_wallpaper");
+}