Allow a wrapping build that checks this source tree out as a
sub-package to stamp its own release tag and commit id onto the UI
(otherwise the version panel reports the host tree's git HEAD,
which the wrapper doesn't control).
GPUSTACK_UI_VERSION overrides the release tag and GPUSTACK_UI_COMMIT_ID
overrides the short commit id; both fall back to the git tag /
commit at HEAD when unset, preserving existing behavior. Names are
namespaced to avoid colliding with the many tools and CI runners
that already set a generic VERSION.
