Railbase core v0.5.26

CLI: auto-offer a free port

When railbase serve's HTTP port is already in use, an interactive terminal now offers to start on the next free port (e.g. :8095 → :8096) instead of only printing the lsof/kill recovery hint. Press Enter (or y) and Railbase boots on the new port.

Non-interactive runs (CI, systemd, piped stdin) keep the original fail-fast behaviour, so automation never silently lands on an unexpected port.

Builds on v0.5.25 (plugin RBAC unification + admin UI conformance + i18n).