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/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "wechselbild"
+version = "0.1.0"
+description = "Unbegrenzter Sperrbildschirm-Wechsel für Android"
+authors = ["Wechselbild"]
+edition = "2021"
+
+[lib]
+name = "wechselbild_lib"
+crate-type = ["staticlib", "cdylib", "rlib"]
+
+[build-dependencies]
+tauri-build = { version = "2", features = [] }
+
+[dependencies]
+tauri = { version = "2", features = [] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tauri-plugin-wallpaper = { path = "../plugins" }
+
+[features]
+default = ["custom-protocol"]
+custom-protocol = ["tauri/custom-protocol"]