Add user account settings page with profile management

- Add /account route with profile form (firstName, lastName, address, IBAN)
- Create UserProfile type and MongoDB users collection
- Implement server actions for getting and updating user profile
- Add Account Circle icon to PageHeader linking to /account
- Install Material UI icons for account icon
- Add form input disabling during save with loading spinner
- Add cancel button to discard changes and return home
- Add English and Croatian translations for account page
- Update locale names with flag emojis in language selector

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

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

package.json

@@ -9,7 +9,11 @@
     "seed": "node -r dotenv/config ./scripts/seed.js"
   },
   "dependencies": {
+    "@emotion/react": "^11.14.0",
+    "@emotion/styled": "^11.14.1",
     "@heroicons/react": "^2.0.18",
+    "@mui/icons-material": "^7.3.5",
+    "@mui/material": "^7.3.5",
     "@tailwindcss/typography": "^0.5.10",
     "@types/node": "20.5.7",
     "@zxing/browser": "^0.1.4",