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

sw.js

@@ -1,10 +1,9 @@
-var CACHE_NAME = 'xmrpay-v2';
+var CACHE_NAME = 'xmrpay-v3';
 var ASSETS = [
   '/',
   '/index.html',
   '/app.js',
   '/i18n.js',
-  '/monitor.js',
   '/style.css',
   '/lib/qrcode.min.js'
   // xmr-crypto.bundle.js is lazy-loaded and runtime-cached