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/build.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -e
 
-VERSION="0.3.8"
+VERSION="$(cat "$(dirname "$0")/VERSION")"
 APP_NAME="studioE5"
 BIN_NAME="studioE5-agent"
 LDFLAGS="-X main.version=${VERSION}"