chore(01-01): configure Vitest test infrastructure

- Add vitest.config.js with jsdom environment and global test APIs
- Install vitest and jsdom as dev dependencies
- Add test scripts (test, test:run) to package.json

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -0,0 +1,20 @@
+{
+  "name": "expense-splitter",
+  "version": "1.0.0",
+  "description": "A web app for splitting restaurant bills fairly among friends",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build",
+    "preview": "vite preview",
+    "test": "vitest",
+    "test:run": "vitest run"
+  },
+  "devDependencies": {
+    "jsdom": "^28.1.0",
+    "vitest": "^4.0.18"
+  },
+  "dependencies": {
+    "@preact/signals": "^2.8.2"
+  }
+}

vitest.config.js

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    include: ['tests/**/*.test.js'],
+  },
+});