feat(chat-switch): in-memory message cache helper

apps/desktop/src/lib/messageMemoryCache.ts

@@ -0,0 +1,37 @@
+// In-memory cache of the most-recently-rendered messages for each
+// conversation. Survives React component unmount/remount (used by
+// `useConversationMessages` to initialize state synchronously when
+// ConversationPage is remounted on chat switch). Session-scoped — lost
+// on full app reload. The SQLite cache (`messageCache.ts`) is still the
+// source of truth for cross-session persistence; this layer just shaves
+// off the round-trip-to-disk spinner flash.
+//
+// Two-tier semantics:
+//   * `hasCachedMessages(id)` returns true even for a known-empty chat
+//     so the hook can suppress the loading spinner on re-entry.
+//   * `getCachedMessages(id)` returns a defensive copy so callers can't
+//     mutate the cached array.
+
+import type { DecryptedMessage } from '@chat-app/shared/chat';
+
+const cache = new Map<string, DecryptedMessage[]>();
+
+export function getCachedMessages(conversationId: string): DecryptedMessage[] {
+  const stored = cache.get(conversationId);
+  return stored ? stored.slice() : [];
+}
+
+export function hasCachedMessages(conversationId: string): boolean {
+  return cache.has(conversationId);
+}
+
+export function setCachedMessages(
+  conversationId: string,
+  messages: DecryptedMessage[],
+): void {
+  cache.set(conversationId, messages.slice());
+}
+
+export function __resetForTests(): void {
+  cache.clear();
+}