perf: implement LRU-cached persistent feed mounting to eliminate Virtuoso flash

Keep Board/Catalog feeds mounted outside React Router's control to prevent
Virtuoso from remounting when navigating to/from post pages. The feed is
hidden via CSS when viewing posts and shown again when returning, eliminating
the visible flash/displacement that occurred during scroll restoration.

- Add FeedCacheContainer to manage LRU cache of 2 feeds
- Modify Board/Catalog to accept cache props and visibility state
- Add route utilities for feed/post route detection

src/components/feed-cache-container/feed-cache-container.module.css

@@ -0,0 +1,13 @@
+.visible {
+}
+
+.hidden {
+  visibility: hidden;
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 100%;
+  pointer-events: none;
+  overflow: hidden;
+  height: 0;
+}

src/components/feed-cache-container/feed-cache-container.tsx

@@ -0,0 +1,99 @@
+import { useEffect } from 'react';
+import { useLocation } from 'react-router-dom';
+import useFeedCacheStore, { CachedFeed } from '../../stores/use-feed-cache-store';
+import { getFeedCacheKey, getFeedType, isFeedRoute } from '../../lib/utils/route-utils';
+import Board from '../../views/board';
+import Catalog from '../../views/catalog';
+import styles from './feed-cache-container.module.css';
+
+interface FeedContextFromKey {
+  viewType: 'all' | 'subs' | 'mod' | 'board';
+  boardIdentifier?: string;
+  timeFilterName?: string;
+}
+
+const parseFeedKey = (key: string): FeedContextFromKey => {
+  const segments = key.split('/').filter(Boolean);
+
+  const filteredSegments = segments.filter((s) => s !== 'catalog');
+  if (filteredSegments[0] === 'all') {
+    return {
+      viewType: 'all',
+      timeFilterName: filteredSegments[1],
+    };
+  }
+  if (filteredSegments[0] === 'subs') {
+    return {
+      viewType: 'subs',
+      timeFilterName: filteredSegments[1],
+    };
+  }
+  if (filteredSegments[0] === 'mod') {
+    return {
+      viewType: 'mod',
+      timeFilterName: filteredSegments[1],
+    };
+  }
+
+  return {
+    viewType: 'board',
+    boardIdentifier: filteredSegments[0],
+    timeFilterName: filteredSegments[1],
+  };
+};
+
+interface CachedFeedWrapperProps {
+  feed: CachedFeed;
+  isVisible: boolean;
+}
+
+const CachedFeedWrapper = ({ feed, isVisible }: CachedFeedWrapperProps) => {
+  const context = parseFeedKey(feed.key);
+
+  return (
+    <div className={isVisible ? styles.visible : styles.hidden}>
+      {feed.type === 'catalog' ? (
+        <Catalog
+          feedCacheKey={feed.key}
+          viewType={context.viewType}
+          boardIdentifier={context.boardIdentifier}
+          timeFilterNameFromCache={context.timeFilterName}
+          isVisible={isVisible}
+        />
+      ) : (
+        <Board
+          feedCacheKey={feed.key}
+          viewType={context.viewType}
+          boardIdentifier={context.boardIdentifier}
+          timeFilterNameFromCache={context.timeFilterName}
+          isVisible={isVisible}
+        />
+      )}
+    </div>
+  );
+};
+
+const FeedCacheContainer = () => {
+  const location = useLocation();
+  const { cachedFeeds, accessFeed } = useFeedCacheStore();
+
+  const currentFeedKey = getFeedCacheKey(location.pathname);
+  const isOnFeedRoute = isFeedRoute(location.pathname);
+  const feedType = getFeedType(location.pathname);
+
+  useEffect(() => {
+    if (isOnFeedRoute && currentFeedKey && feedType) {
+      accessFeed(currentFeedKey, feedType);
+    }
+  }, [currentFeedKey, isOnFeedRoute, feedType, accessFeed]);
+
+  return (
+    <>
+      {cachedFeeds.map((feed) => (
+        <CachedFeedWrapper key={feed.key} feed={feed} isVisible={isOnFeedRoute && feed.key === currentFeedKey} />
+      ))}
+    </>
+  );
+};
+
+export default FeedCacheContainer;

src/components/feed-cache-container/index.ts

@@ -0,0 +1 @@
+export { default } from './feed-cache-container';