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/locationActions.ts

@@ -2,7 +2,7 @@
 
 import { z } from 'zod';
 import { getDbClient } from '../dbClient';
-import { BillingLocation, YearMonth } from '../db-types';
+import { BillingLocation, FileAttachment, YearMonth } from '../db-types';
 import { ObjectId } from 'mongodb';
 import { withUser } from '@/app/lib/auth';
 import { AuthenticatedUser } from '../types/next-auth';
@@ -442,8 +442,8 @@ export const fetchAllLocations = withUser(async (user:AuthenticatedUser, year:nu
                     // "bills.hub3aText": 1,
                     // project only file name - leave out file content so that
                     // less data is transferred to the client
-                    "utilBillsProofOfPaymentUploadedAt": 1,
-                    "utilBillsProofOfPaymentAttachment.fileName": 1,
+                    "utilBillsProofOfPayment.fileName": 1,
+                    "utilBillsProofOfPayment.uploadedAt": 1,
                 },
             },
             {
@@ -505,7 +505,7 @@ export const fetchLocationById = async (locationID:string) => {
                 projection: {
                     // don't include the attachment binary data in the response
                     "bills.attachment.fileContentsBase64": 0,
-                    "utilBillsProofOfPaymentAttachment.fileContentsBase64": 0,
+                    "utilBillsProofOfPayment.fileContentsBase64": 0,
                 },
             }
         );
@@ -599,7 +599,7 @@ export const setSeenByTenantAt = async (locationID: string): Promise<void> => {
  * @param file - The file to serialize
  * @returns BillAttachment object or null if file is invalid
  */
-const serializeAttachment = async (file: File | null) => {
+const serializeAttachment = async (file: File | null):Promise<FileAttachment | null> => {
     if (!file) {
         return null;
     }
@@ -625,11 +625,12 @@ const serializeAttachment = async (file: File | null) => {
         fileType,
         fileLastModified,
         fileContentsBase64,
+        uploadedAt: new Date(),
     };
 }
 
 /**
- * Uploads utility bills proof of payment attachment for a location
+ * Uploads a single proof of payment for all utility bills in a location
  * @param locationID - The ID of the location
  * @param formData - FormData containing the file
  * @returns Promise with success status
@@ -639,23 +640,32 @@ export const uploadUtilBillsProofOfPayment = async (locationID: string, formData
 
     try {
 
-        // check if attachment already exists for the location
-        const dbClient = await getDbClient();
+        // First validate that the file is acceptable
+        const file = formData.get('utilBillsProofOfPayment') as File;
 
-        const existingLocation = await dbClient.collection<BillingLocation>("lokacije")
-            .findOne({ _id: locationID }, { projection: { utilBillsProofOfPaymentAttachment: 1 } });
-
-        if (existingLocation?.utilBillsProofOfPaymentAttachment) {
-            return { success: false, error: 'An attachment already exists for this location' };
+        // validate max file size from env variable
+        const maxFileSizeKB = parseInt(process.env.MAX_PROOF_OF_PAYMENT_UPLOAD_SIZE_KB || '1024', 10);
+        const maxFileSizeBytes = maxFileSizeKB * 1024;
+        
+        if (file && file.size > maxFileSizeBytes) {
+            return { success: false, error: `File size exceeds the maximum limit of ${maxFileSizeKB} KB` };
         }
 
-        const file = formData.get('utilBillsProofOfPaymentAttachment') as File;
-
         // Validate file type
         if (file && file.type !== 'application/pdf') {
             return { success: false, error: 'Only PDF files are accepted' };
         }
 
+        // check if attachment already exists for the location
+        const dbClient = await getDbClient();
+
+        const existingLocation = await dbClient.collection<BillingLocation>("lokacije")
+            .findOne({ _id: locationID }, { projection: { utilBillsProofOfPayment: 1 } });
+
+        if (existingLocation?.utilBillsProofOfPayment) {
+            return { success: false, error: 'An attachment already exists for this location' };
+        }
+
         const attachment = await serializeAttachment(file);
 
         if (!attachment) {
@@ -667,8 +677,9 @@ export const uploadUtilBillsProofOfPayment = async (locationID: string, formData
             .updateOne(
                 { _id: locationID },
                 { $set: {
-                    utilBillsProofOfPaymentAttachment: attachment,
-                    utilBillsProofOfPaymentUploadedAt: new Date()
+                    utilBillsProofOfPayment: {
+                        ...attachment
+                    },
                 } }
             );