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/gen/android/build.gradle.kts

@@ -0,0 +1,22 @@
+buildscript {
+    repositories {
+        google()
+        mavenCentral()
+    }
+    dependencies {
+        classpath("com.android.tools.build:gradle:8.11.0")
+        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.25")
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+tasks.register("clean").configure {
+    delete("build")
+}
+