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/LanguageSwitcher.tsx

@@ -78,7 +78,7 @@ export default function LanguageSwitcher({
               setOpen(false);
             }}
             label={l.label}
-            className="w-full flex items-center justify-between gap-2 text-sm px-2 py-2 rounded-lg text-muted outline-hidden transition hover:text-fg hover:bg-card data-active:bg-[color-mix(in_srgb,var(--accent)_24%,transparent)] data-active:text-fg"
+            className="w-full flex items-center justify-between gap-2 text-sm px-2 py-2 rounded-lg text-muted outline-hidden transition hover:text-fg hover:bg-card"
           >
             <span>{l.label}</span>
             {l.code === value && <Check className="w-4 h-4 text-accent" />}