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/lib/db-types.ts

@@ -14,6 +14,20 @@ export interface YearMonth {
     month: number;
 };
 
+/** User profile data */
+export interface UserProfile {
+    /** user's ID */
+    userId: string;
+    /** first name */
+    firstName?: string | null;
+    /** last name */
+    lastName?: string | null;
+    /** address */
+    address?: string | null;
+    /** IBAN */
+    iban?: string | null;
+};
+
 /** bill object in the form returned by MongoDB */
 export interface BillingLocation {
     _id: string;