Remove NetworkManager + DBus rom WiFi stack

Following in the spirit of https://github.com/commaai/openpilot/issues/37277. Faster connect times, simpler stack, and tinier AGNOS.

---

Bounty targets
* All of openpilot works
* Works on both comma 3X and comma four
* Zero dbus/jeepney/NetworkManager in the WiFi code path
* At least 10 Discord users run it for a week and sign off on it, with and without tethering

Misc notes:
* Routing priority still needs to work properly (e.g. WiFi > LTE when both are up)
* Doesn't require a new AGNOS; code change can land first, then just drop NM from the next AGNOS build
* On-disk storage can stay as `.nmconnection` for easy rollback and interop with existing saved networks
* I did a quick attempt in https://github.com/commaai/openpilot/pull/37662 that can be used as a reference
* this implementation isn't complete though. for example, if the `ui` process dies, you lose networking.

**Bonus target ($500):** Connect time (to IP) must be strictly faster than NetworkManager on a known WPA2 network, measured over 5 iterations on device.