feat(agent): v0.3.9 sync, UI details, self-update, centralized version

- Add agent/server startup sync (sync/sync_response)
- Centralize agent version in agent/VERSION + expose /api/agent/version
- Display agent version, nodeId and server version in local UI
- Add agent self-update detection/download/restart via helper scripts
- Run start/stop/delete/reset handlers in goroutines to avoid WebSocket blocking
- Update dashboard download links and SUIVI_VPN_ONDEMAND.md
- Document Podman stays installer-managed, not agent-updated

agent/ui.go

@@ -56,6 +56,8 @@ func startUI(dataDir, nodeID, serverAddr string) {
 				return
 			}
 			// Expose a merged view with the agent version for the UI.
+			serverVersion := getServerAgentVersion()
+			updateAvailable := serverVersion != "" && serverVersion != version
 			response := map[string]interface{}{
 				"server":             cfg.Server,
 				"headscale_url":      cfg.HeadscaleURL,
@@ -63,6 +65,8 @@ func startUI(dataDir, nodeID, serverAddr string) {
 				"node_id":            cfg.NodeID,
 				"data_dir":           cfg.DataDir,
 				"version":            version,
+				"server_version":     serverVersion,
+				"update_available":   updateAvailable,
 			}
 			json.NewEncoder(w).Encode(response)
 		case http.MethodPost:
@@ -104,6 +108,34 @@ func startUI(dataDir, nodeID, serverAddr string) {
 		}()
 	})
 
+	http.HandleFunc("/api/update", func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+			return
+		}
+		cfg, _, err := loadOrCreateConfig(dataDir)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		go func() {
+			broadcastUI(map[string]interface{}{
+				"action":  "update_progress",
+				"percent": "10",
+				"message": "Téléchargement de la mise à jour...",
+			})
+			if err := startAgentUpdate(dataDir, cfg.Server); err != nil {
+				log.Printf("Agent update failed: %v", err)
+				broadcastUI(map[string]interface{}{
+					"action":  "update_progress",
+					"percent": "0",
+					"message": "Échec de la mise à jour : " + err.Error(),
+				})
+			}
+		}()
+		w.WriteHeader(http.StatusNoContent)
+	})
+
 	http.HandleFunc("/ws", func(w http.ResponseWriter, r *http.Request) {
 		conn, err := upgrader.Upgrade(w, r, nil)
 		if err != nil {