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>

messages/hr.json

@@ -129,5 +129,23 @@
     "validation": {
       "location-name-required": "Ime nekretnine je obavezno"
     }
+  },
+  "account-page": {
+    "heading": "Postavke Računa"
+  },
+  "account-form": {
+    "title": "Podaci o profilu",
+    "first-name-label": "Ime",
+    "first-name-placeholder": "Unesite svoje ime",
+    "last-name-label": "Prezime",
+    "last-name-placeholder": "Unesite svoje prezime",
+    "address-label": "Adresa",
+    "address-placeholder": "Unesite svoju adresu",
+    "iban-label": "IBAN",
+    "iban-placeholder": "Unesite svoj IBAN",
+    "save-button": "Spremi",
+    "cancel-button": "Odbaci",
+    "success-message": "Profil uspješno ažuriran",
+    "validation": {}
   }
 }