Mini menu client + luminosite impression + endpoints systeme

- Menu client coin haut-gauche : WiFi, luminosite, relancer, redemarrer, eteindre
- Popup WiFi : scan, statut, connexion (reutilise les endpoints existants)
- Popup luminosite : slider -50% a +50%, ajuste la luminosite avant impression
  (ImageEnhance.Brightness dans _preparer_image)
- Endpoints systeme : /api/systeme/eteindre, redemarrer, redemarrer-app
- Endpoint exposition Canon : /api/camera/exposition (GET choix + POST valeur)

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

frontend/css/style.css

@@ -157,6 +157,107 @@ html, body {
   color: rgba(255,255,255,0.4);
 }
 
+/* Mini menu client */
+.btn-menu-client {
+  position: absolute;
+  top: 12px;
+  left: 12px;
+  width: 48px;
+  height: 48px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 1.3rem;
+  color: rgba(255,255,255,0.2);
+  cursor: pointer;
+  border-radius: 50%;
+  transition: color 0.3s;
+  z-index: 60;
+}
+.btn-menu-client:active {
+  color: rgba(255,255,255,0.5);
+}
+.menu-client {
+  position: absolute;
+  top: 60px;
+  left: 12px;
+  background: rgba(0,0,0,0.92);
+  border-radius: 16px;
+  padding: 16px;
+  z-index: 200;
+  min-width: 220px;
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+}
+.menu-client button {
+  display: block;
+  width: 100%;
+  padding: 14px 16px;
+  font-size: 1rem;
+  text-align: left;
+  background: rgba(255,255,255,0.08);
+  color: #fff;
+  border: none;
+  border-radius: 10px;
+  cursor: pointer;
+}
+.menu-client button:active {
+  background: rgba(255,255,255,0.2);
+}
+.menu-client-titre {
+  font-size: .85rem;
+  font-weight: 600;
+  color: rgba(255,255,255,0.5);
+  text-transform: uppercase;
+  letter-spacing: 1px;
+  padding: 0 4px 8px;
+}
+.popup-wifi {
+  position: fixed;
+  inset: 0;
+  background: rgba(0,0,0,0.85);
+  z-index: 300;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+.popup-wifi-inner {
+  background: rgba(30,30,30,0.98);
+  border-radius: 20px;
+  padding: 24px;
+  width: 90%;
+  max-width: 400px;
+}
+.popup-wifi-inner button {
+  padding: 12px;
+  font-size: 1rem;
+  background: rgba(255,255,255,0.1);
+  color: #fff;
+  border: none;
+  border-radius: 10px;
+  cursor: pointer;
+}
+.popup-wifi-inner button:active {
+  background: rgba(255,255,255,0.2);
+}
+.wifi-item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 12px;
+  margin: 4px 0;
+  background: rgba(255,255,255,0.06);
+  border-radius: 10px;
+  cursor: pointer;
+}
+.wifi-item:active {
+  background: rgba(255,255,255,0.15);
+}
+.wifi-item.actif {
+  border-left: 3px solid #4caf50;
+}
+
 /* Popup mot de passe */
 .popup-overlay {
   position: fixed;