quak logout ends the session on the server (closes #108)

logout now calls POST /users/logout with the saved token through the new
Client.logoutOnServer(), then deletes session.json even when that call
fails; in that case it says the server session could not be ended and
exits 1. It prints the account's cache directory and says it still holds
decrypted data. The default cache path moves into
defaultCacheDirectory(), shared with Library.open, so both name the same
directory.

Model: opus-5-5

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: `quak logout` ends the session on the server (issue 108). It calls
+  `POST /users/logout` through the new `Client.logoutOnServer()`, then deletes
+  `session.json` even when that call fails, says so and exits 1. It prints the
+  account's cache directory and says it still holds decrypted data. The default
+  cache path is now `defaultCacheDirectory()` in the library, shared with
+  `Library.open`.
+
 - 2026-09-23: Fixed the backup's per-collection folders (issue 103). Two files
   in one collection with the same title, and two collections with the same name,
   each get their ID added to the name (`IMG_0001 (12345).JPG`, `Trip (10)/`), so
@@ -25,6 +32,7 @@ Tag v1.0.0.
   `originals/` for files no longer in the collection, and the folders of deleted
   or renamed collections, leaving anything else in `collections/` alone. The
   README backup layout states the naming rule.
+
 - 2026-09-23: Checked downloaded originals against their recorded content hash
   (issue 68). `downloadFile`, which `quak get`, the content cache and backup all
   use, hashes the decrypted bytes (unkeyed BLAKE2b-512, standard base64) and
@@ -33,12 +41,14 @@ Tag v1.0.0.
   hashed separately as `<imageHash>:<videoHash>`. `decryptFile` reads older
   clients' `imageHash` and `videoHash` fields for live photos. A file with no
   recorded hash is stored unchecked.
+
 - 2026-09-23: Kept one account's cache from mixing with another's (issue 104).
   When `metadata.json` in the cache directory was written for a different,
   non-zero user ID than the client's, `Library.open` deletes it and `mldata/`
   and starts empty, so the first refresh enumerates from 0. This only happens
   with `--cache-dir` or an explicit `cacheDirectory`; the default path already
   includes the user ID. A test opens one account's cache as another account.
+
 - 2026-09-23: `backup-metadata` no longer stops on one failed ML data request
   (issue 101). Each request of up to 200 files is tried on its own; a failed one
   is logged, its files are written with the reason in `mlDataError`, and the