feat(design): every button on its family, the chip primitive, the lint rule (E28 S3b2)

The second half of the button sweep: the remaining hand-rolled buttons in
feature components, the design lab and the components/ui composites now
stand on Button, IconButton, Pressable, Chip or DragHandle.

- Chip + DismissibleChip (filter / token / tag / suggest / ghost, 24px
  floor, 44 on touch) replace five chip geometries; FilterChipGroup
  renders Chips; overflow lanes measure a mirror painted by chipClass.
- The Plex control row is IconButton's player rung (36, 20px glyph,
  round, overlay; 44 on touch); the YT modal bar is on the sm rung with
  pressed cycle toggles and names that survive a folded label.
- New variants: IconButton ghost-accent, danger-soft; Button warn on a
  new state-warn utility. The .btn-{tone} recipes and btnCls are gone.
- Fixed on the way: membership rows (collection editor, Plex playlist
  add) announce checked/mixed; a one-letter tag chip was 21px wide;
  descriptionLinks moved out of lib/ (it renders components).
- local/button-primitive: a raw <button>/<FocusSafeButton> outside the
  control-primitive files is a lint error (contract-tested, seen red).

frontend/src/components/ui/Button.tsx

@@ -17,6 +17,7 @@ import { FocusSafeButton } from "./FocusSafeButton";
 type ButtonVariant =
   | "primary" // solid accent — the main affirmative action
   | "danger" // solid danger — destructive confirm
+  | "warn" // solid warn — the action of a warning bar (reload for a new version)
   | "outline" // bordered, quiet — secondary action (Full history, Subscribe, Cancel-with-border)
   | "outline-accent" // bordered + accent-tinted — an affirmative secondary (Activate, Approve)
   | "outline-danger" // neutral at rest, danger on hover — a row-level Delete
@@ -43,6 +44,7 @@ const ACT = "enabled:not-aria-disabled:";
 const VARIANTS: Record<ButtonVariant, string> = {
   primary: "font-medium state-filled",
   danger: "font-medium state-danger",
+  warn: "font-medium state-warn",
   outline: `font-medium border border-border text-muted state-layer ${ACT}hover:text-fg`,
   "outline-accent": "font-medium border border-accent/40 text-accent state-layer",
   // Neutral until you reach for it. NOT `danger-soft`, which is already red at rest — the difference