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

README.md

@@ -318,6 +318,7 @@ Endpoints used:
   encrypted token plus key attributes.
 - `POST /users/ott` and `POST /users/verify-email`: email OTP fallback path.
 - `POST /users/two-factor/verify`: TOTP second factor.
+- `POST /users/logout`: end the calling token's session (`quak logout`).
 - `GET /collections/v2?sinceTime=<usec>`: list collections changed since
   microsecond timestamp; pass 0 for a full enumeration.
 - `GET /collections/v2/diff?collectionID=<id>&sinceTime=<usec>`: list files in a
@@ -432,7 +433,9 @@ whatever else fits their use case. `Client.fromJSON(snapshot)` restores a
 working client from that snapshot without re-authenticating; it checks every
 field and each key's length first, and throws an error naming the bad field.
 `client.logout()` clears the token and zeroes the key buffers in place; every
-later call on that client throws.
+later call on that client throws. It does not contact the server, so the token
+stays valid there and in any saved snapshot; `await client.logoutOnServer()`
+first ends the session on the server (`POST /users/logout`).
 
 The CLI stores the snapshot at the platform-appropriate data directory via
 `env-paths`: `~/Library/Application Support/quak/session.json` on macOS,
@@ -442,13 +445,21 @@ you would treat the password itself. A missing file is reported as "not logged
 in"; a file that exists but is corrupt is reported as such, naming the bad
 field. Both exit with status 1.
 
+`quak logout` ends the session on the server, so the token in `session.json`
+stops working even in a copy of the file, and then deletes the file. If the
+server call fails (or the file is corrupt), the file is still deleted, the
+command says the server session could not be ended, and it exits with status 1.
+It does not delete the cache: it prints the account's cache directory and says
+it still holds decrypted data (file keys in `metadata.json`, cached originals
+and thumbnails), for the user to delete if they want it gone.
+
 ### CLI surface
 
 ```
 quak [--cache-dir <path>] <command>             global: local metadata/content cache location
 quak login                                      interactive or QUAK_EMAIL/QUAK_PASSWORD
 quak whoami                                     print logged-in account as JSON
-quak logout                                     delete saved session
+quak logout                                     end the session, delete it
 quak collections [--json]                       list all collections
 quak files --collection <id> [--json]           list files in a collection
 quak get <fileID> [--out path] [--collection]   download and decrypt a file