feat(mobile): in-memory attachment cache by handle id

apps/mobile/lib/attachmentCache.ts

@@ -0,0 +1,18 @@
+// Module-level memo of decrypted-attachment data URLs by handle id.
+// Survives screen unmounts (e.g. user pops in and out of a conversation)
+// but evicts on app restart — good enough for Phase 2; an LRU + disk
+// cache is a later polish.
+
+const cache = new Map<string, string>();
+
+export function getCachedAttachment(id: string): string | undefined {
+  return cache.get(id);
+}
+
+export function setCachedAttachment(id: string, dataUrl: string): void {
+  cache.set(id, dataUrl);
+}
+
+export function clearAttachmentCache(): void {
+  cache.clear();
+}