feat(theme,ui): apply pure white background in light mode and improve accessibility

- Update light mode theme tokens to pure white (#ffffff) background and soft gray borders
- Add supports-[height:100dvh] fallback to ProfessorDetailsDialog max-height
- Add aria-hidden to decorative arrows in Header mobile navigation

refer_landing_page/components/Header.tsx

@@ -255,6 +255,7 @@ export default function Header() {
                       <span className="font-sans text-base">{item.en}</span>
                     </span>
                     <span
+                      aria-hidden="true"
                       className={`text-xs font-mono ${
                         active ? "text-white/80 dark:text-ivory/80" : "text-ink-mute"
                       }`}