Pairing screen #5

Compose screen for first-time setup:

• Server URL (https only).
• Pairing code, sent exactly as typed.
• Device name, prefilled with Build.MODEL and editable. It must be 1-64 characters after trimming. Settings.Global DEVICE_NAME is not used, because it often contains the student's real name.
• Error handling by code: invalid_pairing_code, device_limit_reached (the code is not consumed), invalid_device_name, rate_limited (Retry-After).
• On success, the token is stored through the encrypted token storage exactly once.
• The pairing code and token are never logged.

Depends on the API client and the token storage issues.