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