Hint, ActionRow, Toolbar, EmptyState and ExternalLink replace the muted
text, button row, refresh bar, empty line and _blank anchor that each page
restyled under its own class. ExpandableCard (with a loading twin) holds the
builds and recent-runs cards, which had identical head and body CSS. The
create form's raw checkbox becomes CheckboxField. The components page shows
each new piece.
Closes#3448.
New ui/ primitives: FormField (shared label+control wrapper),
TextField, SelectField, Button — each with a min-height touch target
(2.75em ~= 44px, WCAG 2.5.5) per mara's #3447 ask, and a max-width
instead of a fixed width so the control caps on desktop without
overflowing a narrow/touch viewport.
CreateAgentPage's name field + submit button now come from the kit
instead of page-scoped CSS; ComponentsPage gets a section for each new
primitive with an editable sample.
