Improve participant flow QA safeguards

[Spbd1]

Motivation

• Prevent crashes and 404s during final participant flow caused by stale or partial localStorage and a missing submission route.
• Ensure silent revision-lock/unlock logic and replay behavior remain robust when fields are missing or optional.
• Reduce React hook linter noise from the background autosave and make autosave logic more robust.

Description

• Added a final submission route app/export/submission/page.tsx that renders the existing ExportPanel so /export/submission does not 404.
• Hardened stored-session normalization in utils/session.ts to defensively validate and normalize optional fields (participant profile, pre/post surveys, game/replay objects, revision metadata) so old/stale localStorage shapes yield safe defaults.
• Added helper normalizers and readers (normalizeParticipantProfile, normalizePreRevealSurvey, normalizePostRevealSurvey, and utility readers) to avoid unsafe property access and provide safe defaults for forms and completeness checks.
• Updated components/ParticipantBackgroundForm.tsx autosave effect to use a functional state update to eliminate a stale hook dependency and reliably persist participantProfile.
• Added docs/MANUAL_QA_CHECKLIST.md with step-by-step browser checks for the full participant flow, back-navigation revision behavior, replay, stale localStorage, server submission, and admin exports.

Testing

• Ran npm run typecheck and it completed successfully. ✅
• Ran npm run lint and fixed the earlier hook warning; final lint passed with no errors. ✅
• Ran npm run build and Next.js compiled and statically generated pages successfully. ✅
• Validated the example export with npm run validate:sample (Zod schema) and it succeeded. ✅

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Codex Task