chore: ESLint, Prettier und Vitest einrichten (#001)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

vitest.config.mts

@@ -0,0 +1,12 @@
+import { fileURLToPath } from "node:url";
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  resolve: {
+    alias: { "@": fileURLToPath(new URL("./src", import.meta.url)) },
+  },
+  test: {
+    include: ["tests/**/*.test.ts", "src/**/*.test.ts"],
+    environment: "node",
+  },
+});