fix(popover): stronger, centralized active-row highlight (UAT finding)

The roving-focus row in every popover menu was barely distinguishable from the
menu surface — an accent fill at only 24%, and the recipe was copy-pasted across
5 consumers. Centralize it in PopoverItem (ACTIVE_HIGHLIGHT, applied to every
item) and strengthen it: accent/35 fill + a 1px inset accent ring for a defined
edge that reads across all 6 schemes in both themes. Consumers no longer carry
the recipe.

Uses the now-working `bg-accent/NN` / `ring-accent/NN` utilities (v4 does the
opacity modifier via color-mix) instead of the old hand-written color-mix
arbitrary value the v3 no-op forced. Verified in-browser: fill + inset ring
render on the active item, dark and light.

frontend/src/components/AddToPlaylist.tsx

@@ -124,7 +124,7 @@ export default function AddToPlaylist({
                 // the floating element only). `toggle()` already no-ops while busy, so re-entrancy is
                 // guarded without touching focus; `aria-busy` on the list conveys the pending state.
                 label={playlistName(pl, t)}
-                className="w-full flex items-center gap-2 text-sm px-1.5 py-1.5 rounded-lg text-fg outline-hidden transition hover:bg-card/60 data-active:bg-[color-mix(in_srgb,var(--accent)_24%,transparent)]"
+                className="w-full flex items-center gap-2 text-sm px-1.5 py-1.5 rounded-lg text-fg outline-hidden transition hover:bg-card/60"
               >
                 <span
                   className={`grid place-items-center w-4 h-4 rounded border ${