Add internationalization to account page

Replace hardcoded text with next-intl placeholders for proper i18n support.
- Add translation keys for page title, settings button, and logout button
- Add translations for both Croatian (hr) and English (en) locales

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

messages/en.json

@@ -2,6 +2,11 @@
   "Index": {
     "title": "Welcome!"
   },
+  "account-page": {
+    "title": "User account",
+    "settings-button": "User Settings",
+    "logout-button": "Logout"
+  },
   "PageFooter": {
     "app-description": "Helping you to stay on top of your utility bills",
     "links": {