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/src/models.rs

@@ -0,0 +1,16 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Default, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct WallpaperState {
+  pub image_count: usize,
+  pub enabled: bool,
+  pub shuffle: bool,
+  pub lock_screen_only: bool,
+  pub current_index: usize,
+  pub image_urls: Vec<String>,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SettingRequest { pub name: String, pub value: bool }