Declutters the repo root by consolidating build-common.sh,
build-dist.sh, build-dist-windows.sh, build-dist.ps1, and installer.nsi
into build/. Updates all path references in CI workflows, NSIS installer,
and documentation.
workflow_dispatch-triggered build.yml that produces Windows
installer/portable and Linux tarball as CI artifacts without
creating a release. Trigger from Gitea UI → Actions → Run.