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/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "tauri-plugin-wallpaper"
+version = "0.1.0"
+authors = [ "Wechselbild" ]
+description = ""
+edition = "2021"
+rust-version = "1.77.2"
+exclude = ["/examples", "/dist-js", "/guest-js", "/node_modules"]
+links = "tauri-plugin-wallpaper"
+
+[dependencies]
+tauri = { version = "2.11.3" }
+serde = "1.0"
+thiserror = "2"
+
+[build-dependencies]
+tauri-plugin = { version = "2.6.3", features = ["build"] }