Add test environment configuration for DiffView widget

Add happy-dom test environment and vitest config to enable tests to
run with DOM APIs (window.matchMedia) available.

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

Author: dcrjodle

src/widgets/Ivy.Widgets.DiffView/frontend/package.json

@@ -17,6 +17,7 @@
     "@types/react": "19.2.8",
     "@types/react-dom": "19.2.3",
     "autoprefixer": "10.4.23",
+    "happy-dom": "20.8.9",
     "postcss": "8.5.10",
     "tailwindcss": "3.4.19",
     "typescript": "5.9.3",

src/widgets/Ivy.Widgets.DiffView/frontend/pnpm-lock.yaml

@@ -38,4 +38,7 @@ export default defineConfig({
     sourcemap: true,
     cssCodeSplit: false,
   },
+  test: {
+    environment: "happy-dom",
+  },
 });