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

server/app/api/agent/version/route.ts

@@ -0,0 +1,6 @@
+import { NextResponse } from "next/server";
+import { getAgentVersionInfo } from "@/lib/agent-version";
+
+export async function GET() {
+  return NextResponse.json(getAgentVersionInfo());
+}