Previously `make fetch-gui` extracted the GUI release into cmd/gui/dist/
and the unpacked tree was embedded uncompressed via `//go:embed dist`.
This commits and embeds the GUI bundle (dist.zip) and its release tag
(dist.tag) to the repo so:
- the rclone binary is smaller
- `go build` works on a fresh clone without first running fetch-gui
- a given commit pins an exact GUI version
The "Fetch GUI" step was removed from .github/workflows/build.yml.