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/PlexStatsOverlay.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import type Hls from "hls.js";
 import { X } from "lucide-react";
 import { buildStatRows, type FragMeter, type HlsLevelInfo, type StatRow } from "../lib/plexStats";
+import { IconButton } from "./ui/IconButton";
 
 // "Stats for nerds" debug overlay (E6 S2). Samples the live HTML5 <video> + hls.js state once a second
 // and renders the active mode, resolution, codec/bitrate, dropped frames, buffer, hls level and session
@@ -133,13 +134,9 @@ export function PlexStatsOverlay({
         <span className="text-xs font-semibold uppercase tracking-wide text-white/70">
           {t("plex.player.stats.title")}
         </span>
-        <button
-          onClick={onClose}
-          aria-label={t("plex.player.close")}
-          className="rounded p-0.5 text-white/60 hover:bg-white/10 hover:text-white"
-        >
-          <X className="h-4 w-4" />
-        </button>
+        <IconButton variant="overlay" onClick={onClose} label={t("plex.player.close")}>
+          <X />
+        </IconButton>
       </div>
       {/* No aria-live here: the rows re-sample every second, so a live region would make a screen
           reader announce the changing values continuously and drown out real player announcements.