perf(board): reduce desktop reverse-scroll jank on /all

Apply the asymmetric viewport buffer that already fixed mobile to desktop
multiboard feeds, and trim per-post mount cost so remounts during scroll-up
are cheaper:

- Desktop multiboard buffer goes from {600,600} to {1200,2400} (top-heavy),
  matching the mobile pattern from 99c0bbfac so items stay mounted longer
  when scrolling back up
- Cache feed post height estimate by CID in pretext-height-estimates so
  remounts skip the Pretext text-measurement work
- Memoize CommentMedia and switch its expanded-media-store reads to atomic
  selectors so it stops rerendering on unrelated store changes
- Memoize useCommentMediaInfo return so its reference is stable for memo
  comparators downstream
- Stop useFetchGifFirstFrame from forcing an extra render on every post
  mount by bailing out of equivalent setState calls
- Extract PendingModerationActions out of PostInfo so the two
  usePublishCommentModeration calls only run on the post page when
  mod-approval is actually pending, not on every feed item

src/views/board/board.tsx

@@ -402,7 +402,7 @@ const Board = ({ feedCacheKey, viewType, boardIdentifier: boardIdentifierProp, i
   const virtuosoRef = useRef<VirtuosoHandle | null>(null);
   const virtuosoStateKey = feedCacheKey ? `${feedCacheKey}-${BOARD_SORT_TYPE}` : `${location.pathname}-${BOARD_SORT_TYPE}`;
   const navigationType = useNavigationType();
-  const boardViewportBuffer = isMultiboardView ? (isMobile ? { bottom: 1400, top: 2400 } : { bottom: 600, top: 600 }) : { bottom: 1200, top: 1200 };
+  const boardViewportBuffer = isMultiboardView ? (isMobile ? { bottom: 1400, top: 2400 } : { bottom: 1200, top: 2400 }) : { bottom: 1200, top: 1200 };
   const boardMinOverscanItemCount = isMultiboardView && isMobile ? { bottom: 4, top: 8 } : undefined;
 
   const boardItemContent = useCallback(