fix: wait for NM auto-reconnect before starting AP on reboot

After WiFi is configured and device reboots, NetworkManager needs
a few seconds to auto-connect to saved WiFi. Without waiting,
AP starts immediately and occupies wlan0, preventing NM reconnect.

- Add hasSavedWifiConnection() to check for saved WiFi profiles
- Wait up to 20s for NM auto-connect before falling back to AP
- First boot (no saved WiFi) still starts AP immediately
