feat: multi-device support — unique service UUID, identify-on-reconnect, broadcast challenge

[HMAKT99]

Summary

Fixes three root causes that together made multi-phone scenarios broken:

1. Per-Mac unique BLE service UUID — each Mac generates a UUID on first run (DaemonConfig), stored in ~/Library/Application Support/TouchBridge/config.json. Phones now only connect to their paired Mac, not every TouchBridge Mac in range.
2. Identify-on-reconnect — paired device sends an encrypted identify message after ECDH; daemon recognises it without re-pairing. Fixes auth always failing after Mac reboot (deviceID was lost from session).
3. Broadcast challenges — all identified paired devices get challenged simultaneously; first valid response wins. Others expire naturally.

Re-raised: was PR #16, auto-closed when base branch was deleted after merging PR #15.

Test plan

• Pair two iPhones to same Mac — both get prompted on sudo
• Reboot Mac, reconnect — auth works without re-pairing
• Two Macs running TouchBridge nearby — phone only connects to its paired Mac