No disk-space guard or in-app rollback for the ESPHome toolchain update

VERIFIED LIVE 2026-09-06: POST /settings/esphome/toolchain/rollback {target: 2026.6.5} rolled the host venv back from 2026.8.2 in 50 s via the host helper with the dashboard down (build_backend stays venv-host, dashboard_up=false) and brought :6052 back. Disk guard: the helper refuses under 1.5 GiB (Pi has 403 GB free, so exercised by unit test only). rollback_target now also covers a failed update that still moved the venv, and never offers a release past the dashboard boundary. — ORIGINAL: no disk guard, rollback only as a shell command in the job log.