Add board support: NANO_G2_ULTRA (hwModel 18)

[jamesarich]

Meshtastic-Android's own bootloader-OTA quirks manifest (androidApp/src/main/assets/device_bootloader_ota_quirks.json) flags NANO_G2_ULTRA (hwModel 18, PlatformIO target nano-g2-ultra) as requiresBootloaderUpgradeForOta: true, but there's no corresponding board in src/boards/ here.

Reference: https://github.com/meshtastic/Meshtastic-Android/blob/main/androidApp/src/main/assets/device_bootloader_ota_quirks.json

Needs someone with the physical hardware to bring up a src/boards/nano_g2_ultra port (UF2_BOARD_ID, VID/PID, pin defs) against firmware's nano-g2-ultra variant — not something to guess at without a device to test on.

[b8b8]

I have a few g2 ultras.

[jamesarich]

Draft board port up: #22. Builds clean but is not hardware-tested — everything sourced from meshtastic/firmware's own variant files where possible, but the USB PID and the shared-button-pin behavior specifically need someone with the device to confirm. @b8b8, since you mentioned having a few of these — any chance you could give it a try?