Add quak backup-metadata: dump all decrypted metadata to plain JSON

New command: quak backup-metadata <dir>

Dumps every piece of decrypted account metadata into a directory tree
of plain JSON files without downloading any file content. Layout:

    <dir>/
        account.json                    { email, userID }
        collections/
            <id>-<name>/
                _collection.json        { id, name, type, pubMagicMetadata?, ... }
                <fileID>.json           { id, metadata, magicMetadata?, pubMagicMetadata? }

Also adds collection-level magic metadata decryption (magicMetadata,
pubMagicMetadata, sharedMagicMetadata) to decryptCollection, which was
previously only done for files. The server sends these for visibility
settings, sort order, cover photo selection, etc.

6 new tests covering: account.json, per-collection dirs with
_collection.json, collection pubMagicMetadata decryption, per-file
JSON with all three metadata layers, graceful handling of files with
no magic metadata, and incremental re-run safety. 116 total.

src/model/decrypt.ts

@@ -57,6 +57,9 @@ export const decryptCollection = (
         type: parseCollectionType(raw.type),
         updationTime: raw.updationTime,
         isShared: currentUserID !== undefined && raw.owner.id !== currentUserID,
+        magicMetadata: decryptMagicMetadata(raw.magicMetadata, key),
+        pubMagicMetadata: decryptMagicMetadata(raw.pubMagicMetadata, key),
+        sharedMagicMetadata: decryptMagicMetadata(raw.sharedMagicMetadata, key),
     };
 };