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/stores/use-feed-cache-store.ts

@@ -0,0 +1,54 @@
+import { create } from 'zustand';
+
+export interface CachedFeed {
+  key: string;
+  type: 'board' | 'catalog';
+  lastAccessed: number;
+}
+
+interface FeedCacheState {
+  cachedFeeds: CachedFeed[];
+  maxCacheSize: number;
+  accessFeed: (key: string, type: 'board' | 'catalog') => void;
+  removeFeed: (key: string) => void;
+  isFeedCached: (key: string) => boolean;
+}
+
+const useFeedCacheStore = create<FeedCacheState>((set, get) => ({
+  cachedFeeds: [],
+  maxCacheSize: 2,
+
+  accessFeed: (key: string, type: 'board' | 'catalog') => {
+    const { cachedFeeds, maxCacheSize } = get();
+    const now = Date.now();
+    const existingIndex = cachedFeeds.findIndex((feed) => feed.key === key);
+
+    if (existingIndex !== -1) {
+      const updatedFeeds = [...cachedFeeds];
+      updatedFeeds[existingIndex] = { ...updatedFeeds[existingIndex], lastAccessed: now };
+      set({ cachedFeeds: updatedFeeds });
+    } else {
+      const newFeed: CachedFeed = { key, type, lastAccessed: now };
+      let updatedFeeds = [...cachedFeeds, newFeed];
+
+      if (updatedFeeds.length > maxCacheSize) {
+        updatedFeeds.sort((a, b) => a.lastAccessed - b.lastAccessed);
+        updatedFeeds = updatedFeeds.slice(1);
+      }
+
+      set({ cachedFeeds: updatedFeeds });
+    }
+  },
+
+  removeFeed: (key: string) => {
+    const { cachedFeeds } = get();
+    set({ cachedFeeds: cachedFeeds.filter((feed) => feed.key !== key) });
+  },
+
+  isFeedCached: (key: string) => {
+    const { cachedFeeds } = get();
+    return cachedFeeds.some((feed) => feed.key === key);
+  },
+}));
+
+export default useFeedCacheStore;