Improve proof of payment upload UI with fieldset styling

Changes:
- Wrapped upload section in a styled fieldset with legend
- Adjusted spacing and layout for better visual hierarchy
- Updated translations:
  - Added "upload-proof-of-payment-legend" key
  - English: "Proof of payment"
  - Croatian: "Potvrda o uplati"
  - Updated label text to be more descriptive
  - English: "Here you can upload proof of payment:"
  - Croatian: "Ovdje možete priložiti potvrdu o uplati:"

The upload section now has a consistent fieldset styling
matching other sections of the form.

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

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

messages/hr.json

@@ -68,7 +68,8 @@
       "payment-reference-label": "Poziv na broj:",
       "payment-purpose-code-label": "Šifra namjene:",
       "payment-description-label": "Opis plaćanja:",
-      "upload-proof-of-payment-label": "Priložite potvrdu o uplati:"
+      "upload-proof-of-payment-legend": "Potvrda o uplati",
+      "upload-proof-of-payment-label": "Ovdje možete priložiti potvrdu o uplati:"
     },
     "month-card": {
       "payed-total-label": "Ukupni mjesečni trošak:",