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/PlayerModal.tsx

@@ -46,7 +46,7 @@ import {
   formatViews,
   relativeTime,
 } from "../lib/format";
-import { renderDescription } from "../lib/descriptionLinks";
+import { renderDescription } from "./descriptionLinks";
 import { fullscreenElement } from "../lib/fullscreen";
 import { dispatch, isActivationTarget, useKeyScope } from "../lib/keymap";
 import { useBackToClose } from "../lib/history";
@@ -748,9 +748,9 @@ export default function PlayerModal({
                     href={`https://www.youtube.com/watch?v=${currentVideoId}`}
                     target="_blank"
                     rel="noreferrer"
-                    className="inline-flex items-center justify-center gap-2 px-4 py-2 rounded-xl font-semibold btn-primary transition"
+                    className="inline-flex h-8 items-center justify-center gap-1.5 rounded-lg px-3 text-sm font-medium state-filled touch-target"
                   >
-                    <ExternalLink className="w-4 h-4" />
+                    <ExternalLink className="size-4" />
                     {t("player.openOnYoutube")}
                   </a>
                 </div>