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/SetupWizard.tsx

@@ -6,7 +6,6 @@ import { setLanguage, type LangCode } from "../i18n";
 import LanguageSwitcher from "./LanguageSwitcher";
 import { Button } from "./ui/Button";
 import { inputCls } from "./ui/form";
-import { FocusSafeButton } from "./ui/FocusSafeButton";
 
 // First-run install wizard. Shown by App.tsx while the instance is unconfigured. The one-time
 // setup token comes from the URL the app printed to the container logs (?token=…); every step
@@ -280,27 +279,23 @@ export default function SetupWizard() {
           )}
 
           <div className="flex items-center justify-between mt-6">
-            <FocusSafeButton
+            <Button
+              variant="glass"
+              icon={<ChevronLeft />}
               onClick={() => go(-1)}
               disabled={stepIdx === 0 || busy}
-              className="glass-card glass-hover inline-flex items-center gap-1 px-3 py-2 rounded-xl text-sm disabled:opacity-40 transition"
             >
-              <ChevronLeft className="w-4 h-4" />
               {t("setup.nav.back")}
-            </FocusSafeButton>
-            <FocusSafeButton
+            </Button>
+            <Button
               onClick={next}
-              disabled={busy}
-              className="inline-flex items-center gap-1.5 px-4 py-2 rounded-xl text-sm btn-primary font-medium shadow-md shadow-accent/20 disabled:opacity-50 transition"
+              loading={busy}
+              icon={step === "finish" ? <Check /> : undefined}
+              className="shadow-md shadow-accent/20"
             >
-              {busy ? (
-                <Loader2 className="w-4 h-4 animate-spin" />
-              ) : step === "finish" ? (
-                <Check className="w-4 h-4" />
-              ) : null}
               {step === "finish" ? t("setup.nav.finish") : t("setup.nav.next")}
               {step !== "finish" && !busy && <ChevronRight className="w-4 h-4" />}
-            </FocusSafeButton>
+            </Button>
           </div>
         </div>
       </div>