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/lib/agent-version.ts

@@ -0,0 +1,55 @@
+import fs from "fs";
+import path from "path";
+
+const BIN_NAME = "studioE5-agent";
+
+function findVersionFile(): string | null {
+  // Try a few common paths relative to the server workspace and Next.js build output.
+  const candidates = [
+    path.join(process.cwd(), "..", "agent", "VERSION"),
+    path.join(process.cwd(), "..", "..", "agent", "VERSION"),
+    path.join(process.cwd(), "agent", "VERSION"),
+    path.join(__dirname, "..", "..", "..", "agent", "VERSION"),
+    path.join(__dirname, "..", "..", "agent", "VERSION"),
+    "/app/agent-version",
+  ];
+  for (const candidate of candidates) {
+    if (fs.existsSync(candidate)) {
+      return candidate;
+    }
+  }
+  return null;
+}
+
+export function getAgentVersion(): string {
+  const versionFile = findVersionFile();
+  if (versionFile) {
+    return fs.readFileSync(versionFile, "utf-8").trim();
+  }
+  // Fallback used when the agent workspace is not mounted (should not happen).
+  return "0.3.9";
+}
+
+export interface AgentDownloadUrls {
+  windows: string;
+  windowsZip: string;
+  linux: string;
+  mac: string;
+}
+
+export function getAgentDownloadUrls(version: string): AgentDownloadUrls {
+  return {
+    windows: `/${BIN_NAME}-v${version}.exe`,
+    windowsZip: `/${BIN_NAME}-v${version}-windows.zip`,
+    linux: `/${BIN_NAME}-v${version}`,
+    mac: `/${BIN_NAME}-v${version}-mac`,
+  };
+}
+
+export function getAgentVersionInfo() {
+  const version = getAgentVersion();
+  return {
+    version,
+    downloadUrls: getAgentDownloadUrls(version),
+  };
+}