chore(test): bootstrap vitest + jsdom test infrastructure

Add vitest 4.1, jsdom 25, and @testing-library/preact as devDeps so the
widget gets a real test surface for the upcoming Shadow DOM mount work.

- vitest.config.ts mirrors the build aliases (preact/compat) and uses
  jsdom for DOM-touching tests.
- tests/setup.ts is the place to add polyfills as the surface grows.
- tests/unit/smoke.test.ts confirms vitest runs, jsdom is wired, and
  Shadow DOM API is available.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -7,14 +7,19 @@
     "dev": "vite",
     "build": "vite build",
     "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
     "preact": "^10.19.0"
   },
   "devDependencies": {
-    "vite": "^6.0.0",
+    "@testing-library/preact": "^3.2.4",
+    "@types/node": "^22.0.0",
+    "jsdom": "^25.0.0",
     "typescript": "^5.6.0",
-    "@types/node": "^22.0.0"
+    "vite": "^6.0.0",
+    "vitest": "^4.1.4"
   }
 }

tests/setup.ts

@@ -0,0 +1,2 @@
+// Global test setup for Vitest + jsdom.
+// Add polyfills/stubs here as the test surface grows.

tests/unit/smoke.test.ts

@@ -0,0 +1,21 @@
+import { describe, it, expect } from 'vitest';
+
+describe('Vitest infrastructure', () => {
+  it('runs assertions', () => {
+    expect(1 + 1).toBe(2);
+  });
+
+  it('exposes the jsdom document', () => {
+    const div = document.createElement('div');
+    div.id = 'sanity';
+    document.body.appendChild(div);
+    expect(document.getElementById('sanity')).toBe(div);
+  });
+
+  it('supports Shadow DOM API in jsdom', () => {
+    const host = document.createElement('div');
+    document.body.appendChild(host);
+    const shadow = host.attachShadow({ mode: 'open' });
+    expect(host.shadowRoot).toBe(shadow);
+  });
+});

vitest.config.ts

@@ -0,0 +1,20 @@
+import { defineConfig } from 'vitest/config';
+import { resolve } from 'path';
+
+export default defineConfig({
+  resolve: {
+    alias: {
+      react: 'preact/compat',
+      'react-dom': 'preact/compat',
+      'react-dom/test-utils': 'preact/test-utils',
+      'react/jsx-runtime': 'preact/jsx-runtime',
+      '@': resolve(__dirname, './src'),
+    },
+  },
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./tests/setup.ts'],
+    include: ['tests/unit/**/*.test.ts'],
+  },
+});