feat: replace view-key monitor with TX proof verification

Remove v2 view-key payment monitor (privacy concern — nobody should
enter their private view key on a website). Replace with TX proof
verification where the sender provides TX Hash + TX Key from their
wallet. The proof is cryptographically verified client-side and
stored with the invoice for persistent "Paid" status.

- Remove monitor.js and all view-key monitoring UI/logic
- Add TX proof section: sender enters TX Hash + TX Key
- Client-side verification via check_tx_key equivalent (noble-curves)
- api/verify.php stores/retrieves payment proofs per invoice
- Short URL redirect now includes invoice code for status lookup
- Invoice link shows "Paid" badge once proof is verified
- Deadline badges (7/14/30 days) for payment terms

s.php

@@ -22,7 +22,7 @@ if (!isset($urls[$code])) {
     exit;
 }
 
-$hash = $urls[$code];
+$hash = $urls[$code]['hash'] ?? $urls[$code];
 $base = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] === 'on' ? 'https' : 'http') . '://' . $_SERVER['HTTP_HOST'];
-header('Location: ' . $base . '/#' . $hash, true, 302);
+header('Location: ' . $base . '/#' . $hash . '&c=' . $code, true, 302);
 exit;