Polish export preview readability

docs/CODEX_EXECUTION_LOG.md

@@ -2877,3 +2877,27 @@ Limitations:
 
 Next recommended pass:
 - Continue with report/export preview readability polish, especially large JSON/HTML handoff artifacts.
+
+## Sprint 78 Export preview readability polish (2026-06-20)
+
+Changed:
+- Added preview summary cards for JSON/GeoJSON export payload root type, root key count and rendered preview size.
+- Added a scroll-contained export preview JSON shell and toolbar around the existing stored payload preview.
+- Added wrapping for long JSON keys/values inside the preview body.
+- Added `backend/tests/test_sprint78_export_preview_readability.py`.
+- Updated `frontend/README.md`, `docs/TODO.md` and `CHANGELOG.md`.
+
+Tested:
+- Red step: `python -m pytest backend/tests/test_sprint78_export_preview_readability.py -q` failed on missing preview summary/shell CSS and markup contracts.
+- `python -m pytest backend/tests/test_sprint78_export_preview_readability.py backend/tests/test_sprint77_inspector_mobile_polish.py backend/tests/test_sprint76_export_system_mobile_polish.py backend/tests/test_sprint51_quality_export_polish.py backend/tests/test_sprint64_export_handoff_polish.py backend/tests/test_sprint17_export_foundation.py -q` (`20 passed`)
+- `cd frontend && npm run typecheck`
+- `cd frontend && npm run build`
+
+Open:
+- Run full readiness, redeploy Tower and verify live runtime smoke after deployment.
+
+Limitations:
+- Frontend presentation polish only; no export content, export API, artifact storage, migration, provider fetching or AI/model changes.
+
+Next recommended pass:
+- Continue with accessibility/keyboard focus polish across primary workbench controls.