Fix per-bill proof of payment field name and add environment config

- Updated uploadProofOfPayment to expect 'proofOfPayment' field name
  instead of 'utilBillsProofOfPayment' for semantic clarity
- Removed old not-found.tsx from deprecated route structure
- Added required environment variables for file upload validation:
  - MAX_BILL_ATTACHMENT_UPLOAD_SIZE_KB=1024
  - MAX_PROOF_OF_PAYMENT_UPLOAD_SIZE_KB=1024
- Updated package-lock.json with peer dependency metadata

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

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

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

@@ -1,7 +0,0 @@
-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>
-    );
-}