Initialize PN532 IFD-NFC at startup when configured and detected on I2C

[Copilot]

Description

PN532 smartcard access could remain unavailable after boot when smartcard_interfaces already contained pn532, because activation side-effects were only triggered on value transitions. This change adds startup-time PN532 activation, but only when the NFC interface is configured and a PN532 is detected on I2C.

• Startup activation path

  • Added _maybe_activate_pn532_on_startup(settings) and invoked it during Settings.get_instance() initialization.
  • Activation now happens without requiring a manual NFC test scan.

• I2C-gated hardware detection

  • Added _is_i2c_device_detected() to probe /dev/i2c-1 and /dev/i2c-0 with i2cdetect for address 0x24 (PN532 default).
  • ifdnfc-activate yes is executed only when pn532 is configured and the device probe succeeds.

• Fail-safe behavior

  • No-op when ifdnfc-activate, i2cdetect, or I2C buses are unavailable.
  • Logs activation/probe outcomes; avoids hard failures in non-SeedSignerOS/dev environments.

Example of the new startup gate:

sc_interfaces = settings_obj._data.get(SettingsConstants.SETTING__SMARTCARD_INTERFACES, [])
if "pn532" in sc_interfaces and _is_i2c_device_detected("0x24"):
    subprocess.run(["ifdnfc-activate", "yes"], check=False)

Describe the change simply. Provide a reason for the change.

Include screenshots of any new or modified screens (or at least explain why they were omitted)

No UI changes; screenshots omitted.

This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Documentation
• Other

Checklist

• I’ve run pytest and made sure all unit tests pass before sumbitting the PR

If you modified or added functionality/workflow, did you add new unit tests?

• No, I’m a fool
• Yes
• N/A

I have tested this PR on the following platforms/os:

• Raspberry Pi OS Manual Build
• SeedSigner OS on a Pi0/Pi0W board
• Other

Note: Keep your changes limited in scope; if you uncover other issues or improvements along the way, ideally submit those as a separate PR. The more complicated the PR the harder to review, test, and merge.

[Copilot]

Pull request overview

This PR ensures the PN532 IFD-NFC stack is activated during application startup when PN532 is already configured in settings, avoiding the prior “only activates on value transition” behavior. Activation is gated by a best-effort I2C probe so it only runs when a PN532 is actually present.

Changes:

• Added an i2cdetect-based I2C probe helper (_is_i2c_device_detected) to detect a device at 0x24.
• Added startup-time activation helper (_maybe_activate_pn532_on_startup) that runs ifdnfc-activate yes only when configured + detected.
• Invoked the activation helper during Settings.get_instance() initialization.