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/hr.json

@@ -138,6 +138,7 @@
   "account-form": {
     "title": "Podaci o profilu",
     "info-box-message": "Ovi podaci će se koristiti za generiranje 2D barkoda koji će biti prikazan u pregledu računa, omogućujući podstanarima da ga skeniraju i vrate novac koji ste potrošili na plaćanje režija u njihovo ime.",
+    "warning-missing-data": "Upozorenje: Neki podaci profila nedostaju. 2D barkod neće biti prikazan podstanarima u podijeljenom pregledu računa dok sva polja ne budu popunjena.",
     "first-name-label": "Ime",
     "first-name-placeholder": "Unesite svoje ime",
     "last-name-label": "Prezime",