Validate session snapshots and wipe keys on logout (closes #10)

Client.fromJSON checks every snapshot field and each key's decoded length
and throws an error naming the bad field. toJSON reads the token through a
new ApiClient.getAuthToken and throws when there is none. logout zeroes the
key buffers in place; collectionsSince re-checks for logout after its
request so it never decrypts with zeroed keys. The CLI now reports a
corrupt session file separately from a missing one.

Model: opus-5-5

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-22: Hardened the client session lifecycle (issue 10).
+  `Client.fromJSON` checks every snapshot field and each key's decoded length
+  and names the bad field; `toJSON` reads the token through
+  `ApiClient.getAuthToken` and throws when there is none; `logout` zeroes the
+  key buffers, and `collectionsSince` re-checks for logout after its request so
+  it never decrypts with zeroed keys. The CLI reports a corrupt session file
+  separately from a missing one (`src/cli-session.ts`).
 - 2026-09-22: Sanitized file names taken from server metadata (issue 9). A new
   `src/filename.ts` holds the one sanitizer, used by `quak get`/`get-thumb`
   without `--out`, `downloadFile`/`downloadThumbnail` without `outPath`, and the