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