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/BackToTop.tsx

@@ -2,6 +2,7 @@ import { useEffect, useRef, useState } from "react";
 import { createPortal } from "react-dom";
 import { ArrowUp } from "lucide-react";
 import { useTranslation } from "react-i18next";
+import { Pressable } from "./Pressable";
 
 // A floating "back to top" button that fades in once the current page's scroll container is
 // scrolled past a threshold, and smooth-scrolls it back to the top. Every page scrolls inside a
@@ -50,7 +51,8 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
   // commonly hide floating corner buttons via attribute selectors like [aria-label="Back to top"].
   // The accessible name comes from a visually-hidden child instead, which those selectors can't match.
   return createPortal(
-    <button
+    <Pressable
+      align="center"
       onClick={toTop}
       className={`fixed bottom-5 right-5 z-chrome inline-flex items-center justify-center w-16 h-16 rounded-full glass-card glass-hover text-fg hover:text-accent transition-all duration-200 ${
         show ? "opacity-100 translate-y-0" : "opacity-0 translate-y-3 pointer-events-none"
@@ -58,7 +60,7 @@ export default function BackToTop({ dep, threshold = 600 }: { dep?: unknown; thr
     >
       <ArrowUp className="w-7 h-7" aria-hidden="true" />
       <span className="sr-only">{t("common.backToTop")}</span>
-    </button>,
+    </Pressable>,
     document.body,
   );
 }