ux: dark-theme conversion + forum hero + ticker cleanup

- /manufacturers index + DirectoryClient: convert white/light-ink theme
  to BB dark theme so it stops standing out against the rest of the site
- /manufacturers/[slug]: same conversion, includes the new executives
  card grid and Headquarters block
- /nab-2026: same conversion, including hero gradient and sponsor tile
  borders (amber on dark instead of amber on white)
- Forum: remove "Top Performing Threads" widget from both /forum and
  /forum/[slug] (display only — TopThreadsWidget component preserved
  in case admins want to re-enable later)
- Forum hero: vertical padding py-10 → py-5, tagline reduced from
  three sentences to one. Adds a "Sign in to post" or "Start a thread"
  CTA in the hero's top-right depending on auth state.
- Recent Forum Posts ticker: drop replies entirely, surface only thread
  titles so breadth of conversation shows instead of the loudest
  threads dominating with reply chatter.

src/app/forum/[slug]/page.tsx

@@ -361,9 +361,6 @@ export default function ForumCategoryPage() {
             </form>
           )}
 
-          {/* Top Performing Threads Widget (shown when category is loaded) */}
-          {!loading && category && <TopThreadsWidget categoryId={category.id} />}
-
           {/* Search + Sort Bar */}
           <div className="flex flex-col sm:flex-row gap-3 mb-5">
             {/* Search */}