add POS register linkage over LAN

[CDCVM]

Summary

Adds a POS-linked mode alongside the existing standalone keypad. A POS register
drives the terminal over the LAN, while the card tap and the Melon relay stay on
the terminal. Standalone behaviour is unchanged.

See docs/POS-Protocol.md for the full protocol.

Changes

• Add a POS-linked mode, switchable from the header, shown full screen.
• Discover terminals via UDP broadcast (65024); send commands as newline-delimited
  JSON over TCP (65025).
• Report state by polling: every command returns the same status message.
• Support payment, topup, balance, refund_query, refund_execute,
  status, cancel and info.
• Add the kiosk screen: an idle splash, and a transaction face with card-touch
  status rectangles, amount and status text.
• Refuse cancellation once processing has started, to avoid a terminal/POS mismatch.
• Reserve alg and sig in the frame for future authentication or encryption.
• Add docs/POS-Protocol.md and a POS section in the README.

Screenshots

Waiting for card	Payment complete	Insufficient funds

[soltia48]

LGTM