German for 'quack', matching the Ente (German for 'duck') naming. All
references updated: package name, CLI binary, X-Client-Package header,
test descriptions, temp dir prefixes, README, Makefile docker tag.
downloadFile streams the encrypted body from the CDN, buffers it to
the 4 MiB + 17 encrypted chunk boundary, decrypts each chunk via
secretstream pull, and writes the concatenated plaintext to disk.
downloadThumbnail does the same for the thumbnail CDN.
4 unit tests (single-chunk, large single-chunk, filename fallback,
thumbnail) + live integration test that downloads a real 472 KB JPEG
from the dev account and verifies it lands on disk.
Uses mkdtempSync for temp directories (not manual timestamp paths).
4 tests: single-chunk download, metadata-title fallback filename,
multi-chunk stream (50-byte chunks to exercise the buffering/split
logic without allocating 4 MiB), and thumbnail download.
Tests encrypt payloads with sodium's secretstream push, serve them
from a mock fetch, and verify the decrypted file on disk.
