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/design-lab/DesignLab.tsx

@@ -1,9 +1,9 @@
 import { useLayoutEffect, useState } from "react";
-import clsx from "clsx";
 import { SCHEMES, type Scheme } from "../lib/theme";
 import { BatchA } from "./batchA";
 import { BatchB } from "./batchB";
 import { BatchC } from "./batchC";
+import { Chip } from "../components/ui/Chip";
 
 // The design lab (E28): the living style guide. Dev-only — see main.tsx and devOnlyGuard.ts. It sits
 // OUTSIDE <App>, so it owns the theme attributes on <html> itself, through the same data-theme /
@@ -23,12 +23,6 @@ function applyLook(l: Look) {
   el.style.setProperty("--font-scale", String(l.fontScale));
 }
 
-const chip = (on: boolean) =>
-  clsx(
-    "inline-flex items-center gap-1.5 rounded-full border px-2.5 py-1 text-xs transition",
-    on ? "border-accent bg-accent text-accent-fg" : "border-border text-muted hover:text-fg",
-  );
-
 export default function DesignLab() {
   const [look, setLook] = useState<Look>(START);
   useLayoutEffect(() => {
@@ -47,36 +41,22 @@ export default function DesignLab() {
         <strong className="text-sm">Design lab</strong>
         <span className="flex gap-1">
           {(["dark", "light"] as const).map((m) => (
-            <button
-              key={m}
-              type="button"
-              className={chip(look.mode === m)}
-              onClick={() => set({ mode: m })}
-            >
+            <Chip key={m} pressed={look.mode === m} onClick={() => set({ mode: m })}>
               {m}
-            </button>
+            </Chip>
           ))}
         </span>
         <span className="flex flex-wrap gap-1">
           {SCHEMES.map((s) => (
-            <button
-              key={s.id}
-              type="button"
-              className={chip(look.scheme === s.id)}
-              onClick={() => set({ scheme: s.id })}
-            >
+            <Chip key={s.id} pressed={look.scheme === s.id} onClick={() => set({ scheme: s.id })}>
               <span className="h-2.5 w-2.5 rounded-full" style={{ background: s.swatch }} />
               {s.name}
-            </button>
+            </Chip>
           ))}
         </span>
-        <button
-          type="button"
-          className={chip(look.backdrop)}
-          onClick={() => set({ backdrop: !look.backdrop })}
-        >
+        <Chip pressed={look.backdrop} onClick={() => set({ backdrop: !look.backdrop })}>
           backdrop
-        </button>
+        </Chip>
         <label className="flex items-center gap-2 text-xs text-muted">
           text {look.fontScale.toFixed(2)}
           <input