Backend retention now keeps only recent posts, making the time filter UI
and newerThan filtering obsolete. Deleted use-time-filter hook, canonical
multiboard routes only, removed footer suggestions, dropped i18n keys.
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
