Commits ADR-008 (React/Vite admin) for [E01-S01-T07], with the six mandated
sections (Context, Decision, Alternatives, Consequences, Operational impact,
Revisit trigger) per ADR index 46, and adds docs/adr/README.md as the
repository copy of the ADR index (section 70) so the decision-text match is
verifiable inside the repo. Documentation-only: no source, manifest,
lockfile, workflow or test changes.
