Implement logout functionality for account page

- Create LogoutButton client component using signOut from next-auth/react
- Update account page to use LogoutButton instead of static link
- Convert account page to async server component for proper i18n
- Add locale-aware routing for settings link
- Add logging-out-message translations (EN/HR)

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

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

messages/en.json

@@ -5,7 +5,8 @@
   "account-page": {
     "title": "User account",
     "settings-button": "User Settings",
-    "logout-button": "Logout"
+    "logout-button": "Logout",
+    "logging-out-message": "Logging out..."
   },
   "PageFooter": {
     "app-description": "Helping you to stay on top of your utility bills",