fix: download artifacts through the API instead of handing them to Safari

Tapping download opened Artifact.url in the browser, which answered with
"Authorization header is required". That URL is not a web page: git.sr.ht
resolves it to <api origin>/query/artifact/<checksum>/<filename>, which demands
a bearer token. Safari has none and no way to get one, so the download could
never have worked — this predates the upload work.

Fetch it with the client that already holds the token and hand the user the file
through a share sheet. fetchData mirrors fetchText, including its host guard, so
an authenticated request still cannot be aimed anywhere but *.sr.ht over https.

Also guards zero-byte uploads. sr.ht streams into S3, which rejects a zero-part
multipart completion with "MalformedXML: UnknownError" — an error that says
nothing about the cause and cost a round of testing to identify. Empty files are
now refused by name before the request is made.

0 files changed, 0 insertions, 0 deletions