license: force UTC + fixed-Z date format on both sides

Symptom: "Réponse serveur non authentifiée (signature invalide)" on
license activation. Root cause: PHP \DateTimeInterface::ATOM emits the
server's local timezone offset (e.g. +02:00 on a CET host), while the
C# canonicalizer emitted +00:00 after ToUniversalTime(). Same instant,
different string, different bytes, signature mismatch.

Server (ValidateLicense.php):
- All timestamps now built with `new DateTimeZone('UTC')` and formatted
  as 'Y-m-d\TH:i:s\Z' — fixed string, no offset variation.
- Reads issued_at / download_entitlement_until from MySQL as UTC; the
  display is consistent with what the client sees.

Client (LicenseService.cs):
- FormatDateAtom now produces "yyyy-MM-ddTHH:mm:ssZ" with literal Z and
  handles null safely (previous version would have produced "Z" alone
  for a null input thanks to string + null concatenation).

Both sides therefore agree on the canonical bytes for any datetime,
including across daylight savings transitions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/api/routes/ValidateLicense.php

@@ -71,16 +71,22 @@ final class ValidateLicense
             )->execute([$lic['id'], $machineId, $machineLabel ?: null, $now, $now]);
         }
 
-        // Construit la réponse, signe-la, renvoie
-        $entUntil = (new \DateTimeImmutable($lic['download_entitlement_until']))->format(\DateTimeInterface::ATOM);
-        $serverTime = (new \DateTimeImmutable('now'))->format(\DateTimeInterface::ATOM);
+        // Construit la réponse, signe-la, renvoie.
+        // Tous les timestamps sont émis en UTC + format figé "yyyy-MM-ddTHH:mm:ssZ"
+        // pour que la canonicalisation côté client produise EXACTEMENT les mêmes bytes.
+        $utc = new \DateTimeZone('UTC');
+        $fmt = 'Y-m-d\TH:i:s\Z';
+
+        $issuedAt = (new \DateTimeImmutable($lic['issued_at'], $utc))->format($fmt);
+        $entUntil = (new \DateTimeImmutable($lic['download_entitlement_until'], $utc))->format($fmt);
+        $serverTime = (new \DateTimeImmutable('now', $utc))->format($fmt);
         $expired = strtotime($lic['download_entitlement_until']) < time();
 
         $payload = [
             'status'                     => $expired ? 'expired' : 'valid',
             'licenseId'                  => 'lic_' . $lic['id'],
             'ownerName'                  => $lic['owner_name'],
-            'issuedAt'                   => (new \DateTimeImmutable($lic['issued_at']))->format(\DateTimeInterface::ATOM),
+            'issuedAt'                   => $issuedAt,
             'downloadEntitlementUntil'   => $entUntil,
             'maxMachines'                => (int)$lic['max_machines'],
             'serverTime'                 => $serverTime,