feat: AP always-on mode - hotspot stays until WiFi STA connects

Redesign provisioning from one-shot blocking to persistent background manager:
- AP hotspot starts at boot regardless of eth0 status
- Captive portal runs alongside AP for WiFi configuration
- AP automatically shuts down only when WiFi STA connects
- WiFi drops at runtime -> AP auto-restarts
- WiFi connect fails -> AP auto-restarts for retry
- client.js no longer blocks on network; connects WS when ready

Made-with: Cursor

lib/network.js

@@ -173,8 +173,26 @@ function sleep(ms) {
   execSync(`sleep ${ms / 1000}`, { timeout: ms + 2000 });
 }
 
+/**
+ * 检测 wlan0 是否以 STA 模式连接了 WiFi（排除自身热点）
+ */
+function isWifiStaConnected() {
+  const iface = getWifiIface();
+  try {
+    const out = run('nmcli -t -f DEVICE,TYPE,STATE,CONNECTION device');
+    for (const line of out.split('\n')) {
+      const parts = line.split(':');
+      if (parts[0] === iface && parts[1] === 'wifi' && parts[2] === 'connected') {
+        return parts[3] !== CON_NAME;
+      }
+    }
+  } catch (_) {}
+  return false;
+}
+
 module.exports = {
   hasInternet,
+  isWifiStaConnected,
   getWifiIface,
   scanWifi,
   connectWifi,