quak files takes the collection ID as an optional argument (closes #211)

Mandatory arguments are positional, so `quak files` takes the collection ID as an optional argument and the `--collection` option is removed. With a collection ID it lists that collection's files. Without one it lists every file in the account, collection by collection, each file once: the library keeps a record of a file for each collection that holds it, and only the first collection's record is listed, so that collection is its `collectionID` in `--json`. The `--help` text, the README and `TODO.md` say so.

Model: opus-5-5

README.md

@@ -35,8 +35,11 @@ yarn quak login
 # List the user's collections (albums).
 yarn quak collections
 
-# List files in a collection.
-yarn quak files --collection 12345
+# List every file in the account.
+yarn quak files
+
+# List the files in a collection.
+yarn quak files 12345
 
 # Download and decrypt a single file.
 yarn quak get 67890 --out ./photo.jpg
@@ -647,7 +650,7 @@ quak login                                                interactive or QUAK_EM
 quak whoami                                               print logged-in account as JSON
 quak logout                                               end the session, delete it
 quak collections [--json]                                 list all collections
-quak files --collection <id> [--json]                     list files in a collection
+quak files [collectionID] [--json]                        list files in a collection, or every file
 quak get <fileID> [--out path] [--collection]             download and decrypt a file
 quak get-thumb <fileID> [--out] [--collection]            download and decrypt a thumbnail
 quak backup <dir> [--json] [--verify]                     full incremental backup
@@ -676,7 +679,12 @@ answers from what did decrypt (see "Default reads vs. fresh reads").
 `--cache-dir` overrides where the cache lives; without it each account gets its
 own directory under the per-user cache path.
 
-An ID given to `files --collection`, `get`, `get-thumb` or
+`files <collectionID>` lists the files in that collection. `files` with no
+argument lists every file in the account, collection by collection, each file
+once: a file in several collections is listed with the first of them, which is
+its `collectionID` in `--json`.
+
+An ID given to `files`, `get`, `get-thumb` or
 `helper fix-missing-thumbnails --file` is written in decimal digits, no larger
 than 9007199254740991. Anything else, such as `0x10`, `1e3` or `-1`, is refused
 before any request with one line, `Invalid file ID: <arg>` or