feat(updates): auto-apply and self-restart on new dashboard versions

Adds POST /api/updates/apply (pull, rebuild, restart) plus an "Update now"
button and hourly auto-check in the UI, for checkouts that are safely
fast-forwardable. Reintroduces self-restart (previously removed in edc25ca
for cross-environment reliability concerns) per explicit user request.

scripts/restart-helper.js

@@ -0,0 +1,46 @@
+#!/usr/bin/env node
+/**
+ * @file Detached helper spawned by `server/lib/self-restart.js`. Polls the
+ * old dashboard PID until it exits (server/index.js's own SIGTERM handler
+ * has a 5s force-exit backstop, so this never waits forever), then starts a
+ * fresh server from the same entry point and exits itself.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+const { spawn } = require("child_process");
+
+const [, , pidArg, entry] = process.argv;
+const pid = parseInt(pidArg, 10);
+
+// ponytail: fixed poll cadence/cap, not configurable — this script has one caller.
+const POLL_MS = 200;
+const MAX_WAIT_MS = 20_000;
+
+function isAlive(p) {
+  try {
+    process.kill(p, 0);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+function restart() {
+  const child = spawn(process.execPath, [entry], {
+    detached: true,
+    stdio: "ignore",
+    env: process.env,
+  });
+  child.unref();
+  process.exit(0);
+}
+
+function waitAndRestart(waited = 0) {
+  if (isAlive(pid) && waited < MAX_WAIT_MS) {
+    setTimeout(() => waitAndRestart(waited + POLL_MS), POLL_MS);
+    return;
+  }
+  restart();
+}
+
+waitAndRestart();