Browser wizard: switches lose their on/off state in forced-colours mode #956

Found while verifying #954 in Chromium with forced colours emulated (forced-colors: active).

What happens

The browser wizard's switches are checkboxes with appearance: none, drawn with background colours on the box and on its ::after knob. Forced colours replace those backgrounds with the system canvas colour, so a switch shows as an empty outline and "on" looks the same as "off". The range sliders' track is drawn the same way and loses its colour too. Users of Windows high-contrast themes cannot tell which options are enabled.

Acceptance criteria

• In forced-colours mode every switch shows whether it is on, for example with a border and a knob drawn in system colours (ButtonText, Highlight) under @media (forced-colors: active), and the sliders' track and thumb stay visible.
• Disabled (locked) switches remain distinguishable from enabled ones.
• Checked in a browser with forced colours on, as docs/standards/testing.md asks for wizard style changes.