Phase 5 red: collection and file decryption tests

10 tests covering decryptCollection (key + name decryption from raw
server JSON, type mapping, isShared, missing name, wrong key) and
decryptFile (key + metadata decryption, fileType number-to-string
mapping, file/thumbnail header passthrough, wrong key).

Adds src/model/types.ts with both raw (server) and decrypted (library)
type definitions. src/model/decrypt.ts has throwing stubs.

src/model/decrypt.ts

@@ -0,0 +1,23 @@
+// Stub: see the README "Development workflow" section for TDD policy.
+
+import type {
+    Collection,
+    EnteFile,
+    RawCollection,
+    RawEnteFile,
+} from "./types.js";
+
+export const decryptCollection = (
+    _raw: RawCollection,
+    _masterKey: Uint8Array,
+    _currentUserID?: number,
+): Collection => {
+    throw new Error("model.decryptCollection not implemented");
+};
+
+export const decryptFile = (
+    _raw: RawEnteFile,
+    _collectionKey: Uint8Array,
+): EnteFile => {
+    throw new Error("model.decryptFile not implemented");
+};