Add utility bills proof of payment file upload functionality

Changes:
- Updated BillingLocation interface:
  - Added utilBillsProofOfPaymentAttachment field (BillAttachment type)
- Added server action uploadUtilBillsProofOfPayment:
  - Validates PDF file type
  - Serializes file attachment to base64
  - Stores attachment in BillingLocation document
  - Returns success/error status
- Updated ViewLocationCard component:
  - Added file upload input with PDF-only accept
  - Implemented handleFileChange with immediate upload
  - Added upload state management (isUploading, uploadError, attachment)
  - Shows spinner while uploading
  - Input disabled during upload
  - Conditionally renders file input or download link
  - Link displayed after successful upload
- Created route handler for serving proof of payment PDFs:
  - GET /share/proof-of-payment/[id]/route.tsx
  - Fetches attachment from database
  - Converts base64 to binary
  - Returns PDF with proper headers
- Added not-found page for proof of payment route
- Updated middleware to include proof-of-payment in public pages
- Added translations:
  - en: "Upload proof of payment (PDF only)"
  - hr: "Priložite potvrdu o uplati:"

File uploads immediately on selection without page reload.
Only PDF files accepted with client and server-side validation.

🤖 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

@@ -0,0 +1,30 @@
+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 });
+
+    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}`
+        }
+    });
+}