Admin: a segmented control for small unordered choices #1750

Problem

The ffc-control-* family (#1745) covers two values (toggle) and ordered scales of 3 to 11 (steps). A short set of choices with no order between them is still drawn as a <select> or as a stack of radios:

• Settings → General → Dark Mode (Off / On / Auto), a select;
• Settings → General → Code Editor Theme (Auto / Light / Dark), a select;
• form editor → Geofence → Time Behavior (span / daily), two stacked radios with long labels.

A select hides the options behind a click. A rail would also be wrong for these, because it claims "more of the same thing".

Proposal

Add AdminUI::render_segmented(), the family member for 2 to 6 unordered options.

• Markup: a row of native radios drawn as one pill. It posts without a script, and the arrow keys and screen readers work.
• Parameters: name, options, value, default, id, legend, descriptions (only the chosen one shows), size (sm / md / lg), fluid, disabled, and data (autosave attributes on each radio).
• Out of range: below 2 or above 6 options it prints a <select> and calls _doing_it_wrong(), like render_steps().
• States: the family's state language. Hover gets the halo, keyboard focus gets the primary ring, and disabled removes the colour instead of fading.
• Colour: tokens only. The chosen segment is checked against its text in both themes.
• Adoption: the three settings above. Short labels go on the segments, and the explanation moves to a description.

Acceptance criteria

All delivered by #1751.

• render_segmented() with tests: markup, default and fallback values, data, disabled, descriptions, and the out-of-range select with the developer notice.
• CSS in ffc-common.css passes the token, contrast, opacity and spacing guards.
• Dark Mode, Code Editor Theme and Time Behavior use it. The two settings still autosave, and the geofence script still reacts to a time-mode change.
• A guard keeps the three from going back to a select or stacked radios.
• CLAUDE.md "Controls" marks the segmented control as built. CHANGELOG and i18n are updated.

Also in #1751: the steps and segmented controls now keep their layout inside a .form-table. WordPress's fieldset-label rule had pushed each stop's mark 28px to the left of its rail.