Fetch the web bundle the way web-local does

Run 10 got the three jobs it was meant to fix green and then failed on a line
of mine from earlier: `modal volume get … web` answered `[Errno 21] Is a
directory: 'web'`, after the Modal build had already succeeded and written the
bundle.

`modal volume get` wants a destination directory to put `web/` inside, not the
name the copy should take — which is what `just web-local` has always done and
what I did not copy across when this job stopped fetching a desktop tarball and
started fetching a directory. `mkdir -p dist`, fetch into it, upload `dist/web`.

`if-no-files-found: error` on it too, so an empty fetch fails the job rather
than uploading nothing and passing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>