Update public exports and README for Client class

Exports Client, all lower-level modules, and all types from
src/index.ts. Replaces Phase 7 (on-disk session persistence) with
the Client class phase: session lives in the object, consumer
handles persistence via toJSON/fromJSON.

README.md

@@ -649,12 +649,17 @@ Phase 6: download
 - [x] Live integration test: logs in, decrypts collections and files, downloads
       a real JPEG from the dev account and verifies it on disk
 
-Phase 7: session persistence
+Phase 7: Client class
 
-- [ ] `SessionStore` writing an encrypted session blob with a key from the OS
-      keychain (`keytar`) or a `0600` keyfile fallback
-- [ ] `Client.fromSavedSession()` and `Client.saveSession()`
-- [ ] `quack logout` deletes the session and the keychain entry
+- [x] `Client.login({ email, password, totp?, emailOTP? })` performs the full
+      SRP handshake, key unwrap, returns a ready Client
+- [x] `Client.fromJSON(snapshot)` restores from a serialized snapshot
+- [x] `client.toJSON()` produces a `ClientSnapshot` the consumer can persist
+- [x] `client.whoami()`, `client.logout()`
+- [x] `client.listCollections()` with decryption
+- [x] `client.listFiles(collectionID, collectionKey)` with pagination
+- [x] `client.downloadFile(file, outPath?)` and `client.downloadThumbnail()`
+- [x] Literate test/client/usage.test.ts tutorial covering the entire API
 
 Phase 8: CLI