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/PosterTile.tsx

@@ -1,6 +1,7 @@
 import { type ReactNode } from "react";
 import { hueOf, hueTileGradient } from "../lib/hueTile";
 import { navItemAttrs } from "../lib/listNav";
+import { Pressable } from "./ui/Pressable";
 
 // A browse tile split in two: the TOP half shows up to 2 montage posters (from /api/plex/image/{key}),
 // the BOTTOM half is a solid colour-hash band that carries the label — so the text is always readable
@@ -26,12 +27,11 @@ export default function PosterTile({
   const posters = (art ?? []).slice(0, 2);
   const band = hueTileGradient(hueOf(title));
   return (
-    <button
-      type="button"
+    <Pressable
       onClick={onClick}
       title={title}
       {...(navIndex !== undefined && navItemAttrs(navIndex))}
-      className={`glass-hover group relative flex flex-col overflow-hidden rounded-xl text-left transition ${className ?? ""}`}
+      className={`glass-hover group relative flex flex-col overflow-hidden rounded-xl transition ${className ?? ""}`}
     >
       {/* Top half: 2 montage posters, or the colour hash when a group has no art. */}
       <div
@@ -60,6 +60,6 @@ export default function PosterTile({
         <div className="truncate text-sm font-semibold text-white drop-shadow">{title}</div>
         {subtitle != null && <div className="truncate text-2xs text-white/80">{subtitle}</div>}
       </div>
-    </button>
+    </Pressable>
   );
 }