Add quick-add watchlist button and consolidate server actions into lib/actions

Move all server actions from scattered page-level files into shared lib/actions/
directory (settings.ts, titles.ts, watchlist.ts) so they can be reused across
the app. Add a hover-triggered plus button on explore page title cards that lets
users add titles to their watchlist without navigating away.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/(pages)/explore/title-row.tsx

@@ -73,6 +73,7 @@ export function TitleRow({ heading, icon, items }: TitleRowProps) {
                     releaseDate={item.releaseDate}
                     voteAverage={item.voteAverage}
                     href={`/titles/tmdb-${item.tmdbId}-${item.type}`}
+                    showQuickAdd
                   />
                 </motion.div>
               </CarouselItem>