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/lib/db-types.ts

@@ -69,6 +69,8 @@ export interface BillingLocation {
     rentAmount?: number | null;
     /** (optional) whether the location has been seen by tenant */
     seenByTenant?: boolean | null;
+    /** (optional) utility bills proof of payment attachment */
+    utilBillsProofOfPaymentAttachment?: BillAttachment|null;
 };
 
 export enum BilledTo {