SelectField and the refresh-interval picker are built on a new Listbox, a
select-only combobox: a button that keeps focus and a listbox in the top
layer, so it works the same in every current browser, is not clipped by a
scrolling ancestor and shows above a modal dialog. Keys, type-ahead and
placement are pure functions with tests. required becomes aria-required, as a
button has no native form validation.
Per mara's review on this PR: the labelled SelectField-based picker
read as way too heavy for a passive-until-touched setting - a full
label+bordered-control form field row above the table. Replace it with
a compact inline "clock icon - value - chevron" control (a native
<select> still drives the interaction, just stripped of SelectField/
FormField's chrome), quiet until hovered/focused the same way
LinksMenu's header button is.
Also move it out of the panel body entirely: Panel gains an `actions`
slot in its title row (right-aligned via margin-left: auto), so the
picker sits next to the "hives" heading instead of taking its own row
and pushing the table down - per the design guide's own "a control
belongs next to the thing it affects" rule.
Verified with headless chromium screenshots (full page + a tight
close-up crop) against the built bundle: the picker now reads as
"(clock) 30s (chevron)" inline with the panel title, no extra vertical
space taken from the table. Added a Panel "with actions" demo to
ComponentsPage.
