Prepare v0.4.0 stable candidate

[FanyinLiu]

Summary

• promote the v0.4.0 beta line to a stable candidate package/docs state
• harden quiet-observation session provenance so restored browser/renderer state cannot replay stale companion summaries
• update release notes, localized README entry points, in-app release spotlight, and release gates for v0.4.0 stable

Verification

• npm run verify:release
• npm run prerelease-check -- v0.4.0 --skip=A
• npm run prerelease-check -- v0.4.0 --only=A (A.1-A.5 pass; A.6-A.7 pending until merged to origin/main with CI)
• npm run i18n:audit

Release note

Do not tag v0.4.0 from this branch. Merge to main, wait for CI, rerun Stage A, then tag/push v0.4.0.