서버, 클라이언트, 서비스 워커 공용 코드를 lib/shared로 분리하고 브라우저 전용 모듈을 lib/client로 재구성

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/shared/serviceWorker.ts

@@ -0,0 +1,27 @@
+export const DECRYPTED_FILE_URL_PREFIX = "/_internal/decryptedFile/";
+
+export interface FileMetadata {
+  isLegacy: boolean;
+  dataKey: CryptoKey;
+  encContentSize: number;
+  contentType: string;
+}
+
+export interface DecryptionPrepareMessage extends FileMetadata {
+  type: "decryption-prepare";
+  fileId: number;
+}
+
+export interface DecryptionReadyMessage {
+  type: "decryption-ready";
+  fileId: number;
+}
+
+// Sent with a MessagePort to which the client replies with `FileMetadata | null`
+export interface DecryptionMetadataRequestMessage {
+  type: "decryption-metadata-request";
+  fileId: number;
+}
+
+export type ServiceWorkerMessage = DecryptionPrepareMessage;
+export type ServiceWorkerResponse = DecryptionReadyMessage | DecryptionMetadataRequestMessage;