feat(ui): light/dark theme toggle (PR #7)

Update Gradio SDK to 6.18.0 (resolves #5)

Fix dark-mode text contrast in report

Redesign input form to match report identity

Redesign report: pre-publish readiness panel

UI: immediate loading feedback + button lock during audit

Post Audit MVP — Gradio Space (hybrid rules + Gemma 4 E4B on Modal)