🧪 Add Vitest testing framework with basic composition test

Set up testing infrastructure using Vitest and React Testing Library.
Add test verifying empty MyComposition renders without errors.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

videos/vitest.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react";
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: "jsdom",
+    setupFiles: ["./src/test/setup.ts"],
+    include: ["src/**/*.test.{ts,tsx}"],
+  },
+});