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/PlaylistLimitsHint.tsx

@@ -4,6 +4,7 @@ import { Info } from "lucide-react";
 
 import { PopoverSurface } from "./ui/AnchoredPopover";
 import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
+import { Button } from "./ui/Button";
 
 // The playlists module shows fewer lists than youtube.com does, and without a word about it that
 // reads as Siftlode being broken (the owner's ask, 2026-09-03: say it is YouTube's API, up front).
@@ -29,14 +30,9 @@ export default function PlaylistLimitsHint() {
     >
       <Info className="h-3.5 w-3.5 shrink-0" aria-hidden="true" />
       <span>{t("playlists.limits.hint")}</span>
-      <button
-        type="button"
-        ref={pop.refs.setReference}
-        {...pop.getReferenceProps()}
-        className="text-accent hover:underline"
-      >
+      <Button variant="link" ref={pop.refs.setReference} {...pop.getReferenceProps()}>
         {t("playlists.limits.why")}
-      </button>
+      </Button>
       <PopoverSurface
         popover={pop}
         ariaLabel={t("playlists.limits.title")}