Tighten Workbench empty and error states

[smithaldon1]

Summary

• Add status-semantic empty and missing demo-data panels with short Next step recovery copy.
• Distinguish missing local demo state from API connection failures across Overview, Detections, Detection Detail, Recommendations, and RCA/CAPA Draft.
• Stabilize loading panel spacing and extend Playwright coverage for a missing detection route.
• Update Workbench README, troubleshooting docs, and the learning log for the refined recovery behavior.

Closes #177

Test Evidence

• cd apps/web && npm test: 13 passed
• cd apps/web && npm run lint: passed
• cd apps/web && npm run typecheck: passed
• git diff --check: passed
• make lint: passed
• make typecheck: passed
• make docs: passed (Markdown-only docs checker placeholder)
• make test: 195 passed
• make test-e2e: 1 passed
• cd apps/web && npm run build: passed

Docs Impact

• Updated apps/web/README.md to describe Next step recovery text for empty and missing demo-data states.
• Updated docs/demo/TROUBLESHOOTING.md with Workbench missing-state recovery guidance.
• Updated docs/LEARNING_LOG.md with the issue UI: Audit empty, loading, and error states for demo reliability #177 learning summary.

Follow-up Work

• None for this issue. Production incident handling, toast infrastructure, global error-boundary redesign, and real plant failure modes remain out of scope.

[smithaldon1]

Reviewed and approved for merge