Security: Add HMAC validation for short URLs + improve privacy documentation

- Implement HMAC-SHA256 signatures on short URLs to detect server-side tampering
- Add client-side signature verification with hostname-derived secret
- New API endpoint: /api/check-short.php for integrity verification
- Update verify.php with privacy notice (addresses not stored)
- Update README to clarify minimal backend requirement (short URLs, rate caching, proof storage)
- Add toast warning when signature mismatch detected
- Support both old and new format in s.php for backward compatibility
- Update all i18n translations (EN, DE, FR, IT, ES, PT, RU)

Addresses security concern: Server compromise could previously result in address
substitution for short-linked invoices. Now client-side verification detects tampering.

api/check-short.php

@@ -0,0 +1,50 @@
+<?php
+/**
+ * Short URL Integrity Verification API
+ * GET: Return the hash and HMAC signature for client-side verification
+ * 
+ * Security: Allows client-side verification that the hash has not been
+ * tampered with by the server. The signature is verified using the 
+ * hostname as part of the secret HMAC key.
+ */
+
+header('Content-Type: application/json');
+header('Access-Control-Allow-Origin: *');
+
+if ($_SERVER['REQUEST_METHOD'] !== 'GET') {
+    http_response_code(405);
+    echo json_encode(['error' => 'Method not allowed']);
+    exit;
+}
+
+$code = $_GET['code'] ?? '';
+if (empty($code) || !preg_match('/^[a-z0-9]{4,10}$/', $code)) {
+    http_response_code(400);
+    echo json_encode(['error' => 'Invalid code']);
+    exit;
+}
+
+$dbFile = __DIR__ . '/../data/urls.json';
+if (!file_exists($dbFile)) {
+    http_response_code(404);
+    echo json_encode(['error' => 'Invoice not found']);
+    exit;
+}
+
+$urls = json_decode(file_get_contents($dbFile), true) ?: [];
+if (!isset($urls[$code])) {
+    http_response_code(404);
+    echo json_encode(['error' => 'Invoice not found']);
+    exit;
+}
+
+$data = $urls[$code];
+$hash = is_array($data) ? $data['h'] : $data;
+$signature = is_array($data) ? $data['s'] : null;
+
+// Return hash and signature for client-side verification
+echo json_encode([
+    'code' => $code,
+    'hash' => $hash,
+    'signature' => $signature
+]);