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>

src/PSLauncher.Core/Resources/server-pubkey.txt

@@ -1,6 +1 @@
-# Clé publique Ed25519 du serveur PS_Launcher.
-# Génère-la via : `php tools/generate-keypair.php` sur le serveur OVH.
-# Recopie ici les 64 hex chars de public_key_hex (rien d'autre, pas de commentaires sur la même ligne),
-# puis recompile le launcher.
-# Tant que ce fichier commence par '#' ou est vide, la vérification de signature
-# côté client est skippée (le launcher accepte la réponse serveur sans vérifier).
+a32cacfd6f3d52943c6403690fce44bab4933408f49827cb43fa0d3eda267a8f