Component library: skeleton, toast and confirm dialog as ui.ts parts, each in the gallery #5880

Child of #5817. The gallery at /design/components (src/web/gallery.ts) shows what ui.ts has. These parts of the issue's list already exist and need nothing new: empty state (empty), segmented filter (seg), inline field error (field({error})/selectField), copy button (copyField, btn({copy})).

Still missing as ui.ts parts:

• skeleton: a placeholder that holds a list's or card's space while it loads, no layout shift, still under reduced motion.
• toast: today flashToast lives in src/web/layout.ts and three pages write .toast by hand (docs/design/areas/toast.md). Lift it into ui.ts as one part and move those pages onto it.
• confirm dialog: confirmAction links to /-/confirm; the <dialog> markup (docs/design/areas/dialog.md) is drawn by hand in layout.ts and pages/limb.ts. One ui.ts part for the dialog's head/body/foot.

Done when: each is a ui.ts export with a specimen in src/web/gallery.ts (test/design-gallery.test.ts enforces that), and the hand-written copies use it.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.

Parked by the crew: the check stayed red through 2 automatic tries (pull #5897 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.