feat(hero): prev / next arrows on the Mainstage featured strip (#735)

* feat(hero): prev / next arrows on the Mainstage featured strip

Adds left and right chevron buttons over the featured-album hero so a
single click flips to the previous / next album. Hitting the 8 px dot
indicators was awkward and often opened the underlying album by
mistake; the arrows give a generous 44 px touch target on each edge.

- New `goPrev` / `goNext` callbacks wrap with modulo, restart the
  auto-advance timer on click (same pattern the old dot handler used).
- Buttons live in a new `.hero-nav` flex wrapper with `inset: 0` and
  `justify-content: space-between`, so they pin to the hero's left and
  right edges regardless of theme padding. `pointer-events: none` on
  the wrapper + `auto` on the buttons keeps the rest of the hero
  click-through (navigate to album).
- Dots are now decorative spans, not buttons — `pointer-events: none`,
  no hover state, no `onClick`. Clicking near a dot used to navigate
  to the album because the dot was too small to land on.
- i18n: `previousAlbum` / `nextAlbum` keys in all 9 locales.

Reported by zunoz on Discord.

* docs(changelog): note Mainstage hero prev / next arrows (#735)

src/locales/de/hero.ts

@@ -3,4 +3,6 @@ export const hero = {
   playAlbum: 'Album abspielen',
   enqueue: 'Einreihen',
   enqueueTooltip: 'Ganzes Album zur Warteschlange hinzufügen',
+  previousAlbum: 'Vorheriges Album',
+  nextAlbum: 'Nächstes Album',
 };