fix: polish Atelier feedback, empty states and keyboard navigation

[lop-spec]

Scope

• Preserve Atelier and the native HTML/CSS/JavaScript stack; UI-only polish.
• Honest pending/error/history feedback, empty-state recovery, full-catalog test counts and keyboard focus retention.
• Local regression: 101/101 passed; responsive DOM and synthetic interaction checks passed. Screenshot capture timed out, so full visual/native-runtime acceptance is not claimed.
• Cloud-built portable candidate requested for local replacement; no live configuration, credentials, core routing or private data changed.