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/DownloadCenter.tsx

@@ -175,12 +175,9 @@ export default function DownloadCenter() {
         <div className="px-4 pt-2 max-w-4xl w-full mx-auto">
           <div className="flex items-center justify-between gap-3 mb-1">
             <h1 className="text-xl font-semibold">{t("downloads.page.title")}</h1>
-            <button
-              onClick={() => setManage(true)}
-              className="inline-flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-surface transition"
-            >
-              <Settings2 className="w-4 h-4" /> {t("downloads.page.manage")}
-            </button>
+            <Button variant="ghost" size="sm" icon={<Settings2 />} onClick={() => setManage(true)}>
+              {t("downloads.page.manage")}
+            </Button>
           </div>
           <p className="text-sm text-muted mb-4">{t("downloads.page.subtitle")}</p>