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/hooks/use-comment-media-info.ts

@@ -1,4 +1,4 @@
-import { useState, useEffect } from 'react';
+import { useMemo, useState, useEffect } from 'react';
 import { useLocation, useParams } from 'react-router-dom';
 import { getCommentMediaInfo, fetchWebpageThumbnailIfNeeded, CommentMediaInfo } from '../lib/utils/media-utils';
 import { isPendingPostView, isPostPageView } from '../lib/utils/view-utils';
@@ -65,16 +65,15 @@ export const useCommentMediaInfo = (link: string, thumbnailUrl: string, linkWidt
     };
   }, [link, thumbnailUrl, linkWidth, linkHeight, isInPostPageView, isInPendingPostView]);
 
-  const mediaInfo = getCommentMediaInfo(link, thumbnailUrl, linkWidth, linkHeight);
-
-  // Return media info with cached thumbnail dimensions if available
-  if (thumbnailDimensions && mediaInfo) {
-    return {
-      ...mediaInfo,
-      thumbnailWidth: thumbnailDimensions.width,
-      thumbnailHeight: thumbnailDimensions.height,
-    };
-  }
-
-  return mediaInfo;
+  return useMemo(() => {
+    const mediaInfo = getCommentMediaInfo(link, thumbnailUrl, linkWidth, linkHeight);
+    if (thumbnailDimensions && mediaInfo) {
+      return {
+        ...mediaInfo,
+        thumbnailWidth: thumbnailDimensions.width,
+        thumbnailHeight: thumbnailDimensions.height,
+      };
+    }
+    return mediaInfo;
+  }, [link, thumbnailUrl, linkWidth, linkHeight, thumbnailDimensions]);
 };

src/hooks/use-fetch-gif-first-frame.ts

@@ -11,6 +11,10 @@ interface GifFirstFrameState {
   status: GifFirstFrameStatus;
 }
 
+const IDLE_STATE: GifFirstFrameState = { frameUrl: null, status: 'idle' };
+const LOADING_STATE: GifFirstFrameState = { frameUrl: null, status: 'loading' };
+const FAILED_STATE: GifFirstFrameState = { frameUrl: null, status: 'failed' };
+
 const getCachedGifFrame = async (url: string): Promise<string | null> => {
   return await gifFrameDb.getItem(url);
 };
@@ -79,20 +83,20 @@ const parseGif = async (buf: ArrayBuffer): Promise<Blob> => {
 };
 
 const useFetchGifFirstFrame = (url: string | undefined) => {
-  const [gifFirstFrame, setGifFirstFrame] = useState<GifFirstFrameState>({ frameUrl: null, status: 'idle' });
+  const [gifFirstFrame, setGifFirstFrame] = useState<GifFirstFrameState>(IDLE_STATE);
 
   useEffect(() => {
     if (!url) {
-      setGifFirstFrame({ frameUrl: null, status: 'idle' });
+      setGifFirstFrame((prev) => (prev.status === 'idle' ? prev : IDLE_STATE));
       return;
     }
 
     let isActive = true;
-    setGifFirstFrame({ frameUrl: null, status: 'loading' });
+    setGifFirstFrame((prev) => (prev.status === 'loading' ? prev : LOADING_STATE));
 
     const fetchFrame = async () => {
       if (failedUrls.has(url)) {
-        if (isActive) setGifFirstFrame({ frameUrl: null, status: 'failed' });
+        if (isActive) setGifFirstFrame((prev) => (prev.status === 'failed' ? prev : FAILED_STATE));
         return;
       }
 
@@ -119,7 +123,7 @@ const useFetchGifFirstFrame = (url: string | undefined) => {
       } catch (error) {
         failedUrls.add(url);
         console.error('Failed to load GIF frame:', error);
-        if (isActive) setGifFirstFrame({ frameUrl: null, status: 'failed' });
+        if (isActive) setGifFirstFrame((prev) => (prev.status === 'failed' ? prev : FAILED_STATE));
       }
     };