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>

app/ui/PageHeader.tsx

@@ -1,10 +1,14 @@
 import Image from "next/image";
 import Link from "next/link";
 import { SelectLanguage } from "./SelectLanguage";
+import AccountCircleIcon from "@mui/icons-material/AccountCircle";
 
 export const PageHeader = () =>
     <div className="navbar bg-base-100 mb-6">
         <Link className="btn btn-ghost text-xl" href="/"><Image src="/icon3.png" alt="logo" width={48} height={48} /> Režije</Link>
         <span className="grow">&nbsp;</span>
         <SelectLanguage />
+        <Link href="/account/" className="btn btn-ghost btn-circle">
+            <AccountCircleIcon className="w-6 h-6" />
+        </Link>
     </div>