ISSUE-1373: Added wired ethernet only option

[herbenderbler]

Summary

Improves the Wi‑Fi customisation step so users who only need Ethernet can skip Wi‑Fi setup without clearing fields manually or confusing it with Skip customisation (which skips all OS customisation).

Closes #1373

Changes

• Short subtitle explaining that a wired-only setup can skip Wi‑Fi on this screen.
• Use wired Ethernet only button that clears SSID/password/hidden-SSID state, resets secure/open mode appropriately, and continues with the same behaviour as Next with an empty SSID (no Wi‑Fi in the written image; other customisation unchanged).
• Keyboard / screen reader: new focus group so the control fits the existing wizard focus order.

Testing

• Open OS customisation → Wi‑Fi step; use Use wired Ethernet only and confirm the write summary shows no Wi‑Fi when expected.
• Confirm Skip customisation still skips all customisation and jumps to writing as before.