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/.gitignore

@@ -0,0 +1,17 @@
+/.vs
+.DS_Store
+.Thumbs.db
+*.sublime*
+.idea/
+debug.log
+package-lock.json
+.vscode/settings.json
+yarn.lock
+
+/.tauri
+/target
+Cargo.lock
+node_modules/
+
+dist-js
+dist