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/en.json

@@ -68,7 +68,8 @@
       "payment-reference-label": "Reference number:",
       "payment-purpose-code-label": "Purpose code:",
       "payment-description-label": "Payment description:",
-      "upload-proof-of-payment-label": "Upload proof of payment:"
+      "upload-proof-of-payment-legend": "Proof of payment",
+      "upload-proof-of-payment-label": "Here you can upload proof of payment:"
     },
     "month-card": {
       "payed-total-label": "Total monthly expenditure:",