fix: resolve test failures across translations, contact form, and vitest config

- Add 18 missing translation keys to all non-English locales (de, es, fr, it, ja, nl, pt) with English fallback values
- Fix contact form submit test targeting prefix input instead of given_name
- Exclude .next/ build output from vitest to prevent Playwright spec conflicts

vitest.config.ts

@@ -8,7 +8,7 @@ export default defineConfig({
     environment: 'jsdom',
     globals: true,
     setupFiles: ['./vitest.setup.ts'],
-    exclude: ['e2e/**', 'node_modules/**'],
+    exclude: ['e2e/**', 'node_modules/**', '.next/**'],
   },
   resolve: {
     alias: {