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-tauri/tauri.conf.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "productName": "Wechselbild",
+  "version": "0.1.0",
+  "identifier": "de.wechselbild.app",
+  "build": { "beforeDevCommand": "npm run dev", "devUrl": "http://localhost:1420", "beforeBuildCommand": "npm run build", "frontendDist": "../dist" },
+  "app": { "windows": [{ "title": "Wechselbild", "width": 420, "height": 860, "resizable": true }], "security": { "csp": null } },
+  "bundle": { "active": true, "targets": "all", "icon": [] }
+}