Add real-time warning for missing profile data in account form

Added conditional warning alert that displays when required fields are
empty, updating in real-time as the user types:
- Tracks form values with useState and onChange handlers
- Displays warning when firstName, lastName, address, or IBAN is missing
- Warning appears/disappears instantly as user fills or clears fields
- Explains that 2D barcode won't be shown to tenants until all data is complete
- Added translations in English and Croatian

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

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

messages/en.json

@@ -139,6 +139,7 @@
   "account-form": {
     "title": "Profile Information",
     "info-box-message": "This information will be used to generate a 2D barcode displayed in the bill view, allowing tenants to scan it and refund the money you have spent on paying utility bills in their name.",
+    "warning-missing-data": "Warning: Some profile fields are missing. The 2D barcode will not be displayed to tenants on the shared bill view until all fields are filled in.",
     "first-name-label": "First Name",
     "first-name-placeholder": "Enter your first name",
     "last-name-label": "Last Name",