fix: add missing test dependencies to devDependencies

vitest, @testing-library/react, @testing-library/user-event,
@testing-library/jest-dom, jsdom, react, react-dom, @mui/material,
and react-router-dom were all used directly but only available as
transitive dependencies through @kinvolk/headlamp-plugin. pnpm's
strict module resolution prevented them from being resolved.

Also adds process.env.NODE_ENV="test" to vitest config so React
loads its development build (required for act() support in tests).

Fixes #27

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

vitest.config.mts

@@ -1,6 +1,9 @@
 import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
+  define: {
+    'process.env.NODE_ENV': '"test"',
+  },
   test: {
     globals: true,
     environment: 'jsdom',