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/ui/UndoToolbar.tsx

@@ -2,7 +2,7 @@ import { useEffect } from "react";
 import { useTranslation } from "react-i18next";
 import { Redo2, Undo2 } from "lucide-react";
 import { dispatch, useKeyScope } from "../../lib/keymap";
-import { FocusSafeButton } from "./FocusSafeButton";
+import { IconButton } from "./IconButton";
 
 // Reusable undo/redo buttons with Ctrl/Cmd+Z (undo) and Ctrl/Cmd+Y or Ctrl/Cmd+Shift+Z
 // (redo) shortcuts. Pairs with useUndoable but takes plain props, so it works with any
@@ -44,28 +44,14 @@ export default function UndoToolbar({
     return () => window.removeEventListener("keydown", onKey);
   }, [canUndo, canRedo, onUndo, onRedo]);
 
-  const btn =
-    "p-1.5 rounded-lg border border-border text-muted enabled:not-aria-disabled:hover:text-fg enabled:not-aria-disabled:hover:border-accent disabled:opacity-30 transition";
   return (
     <div className="inline-flex items-center gap-1">
-      <FocusSafeButton
-        onClick={onUndo}
-        disabled={!canUndo}
-        title={t("common.undo")}
-        aria-label={t("common.undo")}
-        className={btn}
-      >
-        <Undo2 className="w-4 h-4" />
-      </FocusSafeButton>
-      <FocusSafeButton
-        onClick={onRedo}
-        disabled={!canRedo}
-        title={t("common.redo")}
-        aria-label={t("common.redo")}
-        className={btn}
-      >
-        <Redo2 className="w-4 h-4" />
-      </FocusSafeButton>
+      <IconButton variant="outline" onClick={onUndo} disabled={!canUndo} label={t("common.undo")}>
+        <Undo2 />
+      </IconButton>
+      <IconButton variant="outline" onClick={onRedo} disabled={!canRedo} label={t("common.redo")}>
+        <Redo2 />
+      </IconButton>
     </div>
   );
 }