Admin UI: the two-state toggle joins the ffc-control family #1748

Problem

#1745 introduced the ffc-control-* family and its shared state language (a halo for hover and pressed, the primary ring for focus, a disabled state with no opacity). The two-state toggle predates it and is out of step in three ways:

1. Its class names belong to no family. It uses .ffc-toggle, .ffc-toggle-track and .ffc-toggle-label, while the steps control uses .ffc-control-steps*. CLAUDE.md "Controls" records this as planned.
2. It has no hover or pressed state, and its disabled state fades through opacity: 0.5. That multiplies down contrast, and OpacityContrastTest has to carry it as an exception.
3. A second toggle exists. The user dashboard's notification preferences build their own markup in JS (.ffc-toggle-label + .ffc-toggle-switch) with their own CSS, and hide the checkbox with display: none. That removes it from keyboard focus, so a keyboard user cannot change those preferences.

On/off is also told apart by colour and knob position only.

Acceptance criteria

All delivered by #1749.

• Every toggle the plugin prints (PHP, templates, JS-built markup) uses .ffc-control-toggle, .ffc-control-toggle-track and .ffc-control-toggle-label, and no .ffc-toggle* class is left.
• The user dashboard's notification toggles use the same markup and CSS, and their checkbox is reachable by keyboard; the second implementation is deleted.
• Hover and pressed draw the shared halo; the disabled state uses no opacity, and its OpacityContrastTest entry is removed.
• render_toggle() takes an optional mark parameter (default off) that draws a check or a cross on the knob. Both glyphs come from the Core\Icons registry.
• Autosave keeps working; the badge still anchors after the toggle.
• Tests, CHANGELOG and CLAUDE.md "Controls" and "Settings writes" are updated.