Implement proof of payment download routes

Added two download routes for proof of payment files:

1. Combined route: /share/proof-of-payment/combined/[id]/
   - Downloads location-level proof of payment for all utilities
   - Queries utilBillsProofOfPayment from location
   - Optimized projection for efficient data transfer

2. Per-bill route: /share/proof-of-payment/per-bill/[id]/
   - Downloads proof of payment for individual bills
   - Parses composite ID format: locationID-billID
   - Finds specific bill in location's bills array
   - Returns bill.proofOfPayment

Both routes:
- Return PDF files with proper Content-Type and headers
- Handle 404 for missing locations/bills/proofs
- Use Base64 to binary conversion for file delivery
- Include Last-Modified header for caching
- Use optimized database projections

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

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

app/[locale]/share/proof-of-payment/combined/[id]/not-found.tsx

@@ -0,0 +1,7 @@
+export default function NotFound() {
+    return (
+        <div className="flex min-h-screen flex-col items-center justify-center p-6 bg-base-300">
+            <h2 className="text-2xl font-bold">Proof of payment not found</h2>
+        </div>
+    );
+}