Debian 12 ships golang-go 1.19, which cannot parse go.mod's `toolchain
go1.26.5` directive (unknown directive) — so `install.sh --local-build`
failed on Debian 12 before the panel was ever built. ensure_go only
checked `command -v go` and accepted the broken 1.19.
ensure_go now:
- accepts an existing go only if it is >= 1.21 (the minimum for
toolchain auto-switch that go.mod relies on)
- otherwise installs the distro package, and if that is still too old,
downloads the official Go 1.26.x tarball from go.dev into
/usr/local/go and puts it first on PATH for the installer
Verified on the Debian 12 test host (previously failed with go1.19).
Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
