Intégration station d'impression photo (photostation)

- Icône discrète coin bas-gauche, visible seulement si photostation installée
- Popup PIN (4 chiffres) avant lancement
- Backend : /api/photostation/disponible + /api/photostation/lancer
- kiosk-session.sh : Chromium attend le flag /tmp/photostation_running avant redémarrage
- Script install-photostation.sh pour déploiement sur la Surface

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/css/style.css

@@ -135,6 +135,26 @@ html, body {
   color: rgba(255,255,255,0.4);
 }
 
+.btn-photostation {
+  position: absolute;
+  bottom: 12px;
+  left: 12px;
+  width: 56px;
+  height: 56px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 1.5rem;
+  color: rgba(255,255,255,0.15);
+  cursor: pointer;
+  border-radius: 50%;
+  transition: color 0.3s;
+}
+
+.btn-photostation:hover, .btn-photostation:active {
+  color: rgba(255,255,255,0.4);
+}
+
 /* Popup mot de passe */
 .popup-overlay {
   position: fixed;