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/buildSrc/build.gradle.kts

@@ -0,0 +1,23 @@
+plugins {
+    `kotlin-dsl`
+}
+
+gradlePlugin {
+    plugins {
+        create("pluginsForCoolKids") {
+            id = "rust"
+            implementationClass = "RustPlugin"
+        }
+    }
+}
+
+repositories {
+    google()
+    mavenCentral()
+}
+
+dependencies {
+    compileOnly(gradleApi())
+    implementation("com.android.tools.build:gradle:8.11.0")
+}
+