fix(gallery): correctly show status for downloading OCI images

We can't use the mutate.Extract written bytes as current status as that
will be bigger than the compressed image size. Image manifest don't have
any guarantee of the type of artifact (can be compressed or not) when
showing the layer size.

Split the extraction process in two parts: Downloading and extracting as
a flattened system, in this way we can display the status of downloading
and extracting accordingly.

This change also fixes a small nuance in detecting installed backends,
now it's more consistent and looks if a metadata.json and/or a path with
a `run.sh` file is present.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>