Implement per-bill proof of payment and update field names

Frontend changes:
- Added ViewBillCard proof of payment upload for per-bill mode
  - Conditional rendering based on proofOfPaymentType
  - File upload with PDF validation and loading states
  - Download link to /share/proof-of-payment/per-bill/
- Updated LocationCard to use new utilBillsProofOfPayment field structure

Backend changes:
- Updated locationActions with improved file validation
  - File size validation using MAX_PROOF_OF_PAYMENT_UPLOAD_SIZE_KB
  - PDF type validation before database operations
  - Enhanced serializeAttachment with FileAttachment type
  - Updated database projections for optimized queries
- Updated monthActions to use consolidated field name
- Updated proof-of-payment download route with new field names

Data structure migration:
- Replaced utilBillsProofOfPaymentAttachment + utilBillsProofOfPaymentUploadedAt
  with single utilBillsProofOfPayment object containing uploadedAt
- Consistent use of FileAttachment type across all upload functions

Translations:
- Added upload-proof-of-payment-legend and upload-proof-of-payment-label
  to bill-edit-form section in both English and Croatian

This completes the proof of payment feature implementation for both
combined (location-level) and per-bill modes.

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

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

app/lib/actions/monthActions.ts

@@ -41,8 +41,7 @@ export const addMonth = withUser(async (user:AuthenticatedUser, { year, month }:
             ...prevLocation,
             // clear properties specific to the month
             seenByTenantAt: undefined,
-            utilBillsProofOfPaymentUploadedAt: undefined,
-            utilBillsProofOfPaymentAttachment: undefined,
+            utilBillsProofOfPayment: undefined,
             // assign a new ID
             _id: (new ObjectId()).toHexString(),
             yearMonth: {