Port the quak CLI to the library API (closes #52)

Route every command through Library.open instead of scanning the client.
The read commands (collections, files, get, get-thumb) force a server
round-trip with Library.fresh() before reading, so they answer for
current state, not a stale cache (owner amendment, issue #36).
collections and files list in the library's enumeration order — the
order the pre-library CLI printed, not the newest-first projection — and
present each file from its own decrypted metadata (raw title, microsecond
creationTime). get/get-thumb copy the cached original/thumbnail to --out.
backup, backup-metadata, and the thumbnail helpers are unchanged. A new
global --cache-dir sets the cache location; point commands open with the
background precache off so a one-shot command never downloads the account.

Also addresses #17:
fix-missing-thumbnails reports a non-JPEG image or a video as skipped
(unsupported), distinct from failed, and only a genuine failure exits
non-zero.

Model: opus-4-8

README.md

@@ -420,6 +420,7 @@ you would treat the password itself.
 ### 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
@@ -432,9 +433,20 @@ quak helper list-missing-thumbnails [--json]    find files with missing thumbnai
 quak helper fix-missing-thumbnails [--file ids] generate + upload missing thumbnails
 ```
 
-`get` and `get-thumb` search all collections for the file ID when `--collection`
-is not specified. All listing and backup commands support `--json` for
-machine-readable output.
+Every command reads through the local library cache: the first run fetches the
+account's metadata from the server, and later runs serve from the cache and
+refresh in the background. `--cache-dir` overrides where that cache lives;
+without it each account gets its own directory under the per-user cache path.
+
+`get` and `get-thumb` resolve the file by ID directly, so `--collection` is
+accepted for backward compatibility but ignored. All listing and backup commands
+support `--json` for machine-readable output.
+
+`helper fix-missing-thumbnails` regenerates thumbnails for baseline JPEG images
+only, because the bundled decoder (`jpeg-js`) decodes only JPEG. A non-JPEG
+image (PNG, HEIC) or a video is reported as `skipped` (unsupported format), kept
+distinct from a `failed` repair, and does not affect the exit code; a genuine
+failure still exits non-zero.
 
 ### Backup layout