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/[locale]/share/proof-of-payment/[id]/route.tsx

@@ -1,34 +0,0 @@
-import { getDbClient } from '@/app/lib/dbClient';
-import { BillingLocation } from '@/app/lib/db-types';
-import { notFound } from 'next/navigation';
-
-export async function GET(request: Request, { params:{ id } }: { params: { id:string } }) {
-    const locationID = id;
-
-    const dbClient = await getDbClient();
-    const location = await dbClient.collection<BillingLocation>("lokacije")
-        .findOne({ _id: locationID }, {
-            projection: {
-                utilBillsProofOfPaymentAttachment: 1,
-            }
-        });
-
-    if(!location?.utilBillsProofOfPaymentAttachment) {
-        notFound();
-    }
-
-    // Convert fileContentsBase64 from Base64 string to binary
-    const fileContentsBuffer = Buffer.from(location.utilBillsProofOfPaymentAttachment.fileContentsBase64, 'base64');
-
-    // Convert fileContentsBuffer to format that can be sent to the client
-    const fileContents = new Uint8Array(fileContentsBuffer);
-
-    return new Response(fileContents, {
-        status: 200,
-        headers: {
-            'Content-Type': 'application/pdf',
-            'Content-Disposition': `attachment; filename="${location.utilBillsProofOfPaymentAttachment.fileName}"`,
-            'Last-Modified': `${location.utilBillsProofOfPaymentAttachment.fileLastModified}`
-        }
-    });
-}