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());
+}

server/app/api/download/route.ts

@@ -1,13 +1,12 @@
 import { NextResponse } from "next/server";
-
-const AGENT_VERSION = "0.3.4";
-const AGENT_BIN_NAME = "studioE5-agent";
+import { getAgentVersionInfo } from "@/lib/agent-version";
 
 export async function GET() {
+  const info = getAgentVersionInfo();
   return NextResponse.json({
-    version: AGENT_VERSION,
-    windows: `/${AGENT_BIN_NAME}-v${AGENT_VERSION}.exe`,
-    linux: `/${AGENT_BIN_NAME}-v${AGENT_VERSION}`,
-    mac: `/${AGENT_BIN_NAME}-v${AGENT_VERSION}-mac`,
+    version: info.version,
+    windows: info.downloadUrls.windows,
+    linux: info.downloadUrls.linux,
+    mac: info.downloadUrls.mac,
   });
 }