Refresh landing/demo screenshots and GIF after a real-provider run

## Summary

Known limitation: screenshots and demo GIF should be refreshed after a real-provider recording. Landing still implies mock timings (e.g. 36ms alt text). Distinct from the 60s MP4 in #8.

## Context

- `docs/release/known-limitations.md`
- `scripts/generate-demo-gif.sh` / `scripts/capture-demo-gif.sh` / `scripts/prove-real-voice.sh`
- `public/assets/screenshots/`, `docs/assets/screenshots/`
- `public/landing/index.html` alt text
- PM tasklist Stage 17

## Acceptance criteria

- [ ] New PNG/GIF from a real-provider (or honestly labeled mock if keys unavailable — do not fake 36ms as production STT)
- [ ] Alt text matches the captured UI
- [ ] README/landing still honest about mock vs real

Needs maintainer API keys for the real-provider path.

Start with docs/release/known-limitations.md and inspect scripts/generate-demo-gif.sh, scripts/capture-demo-gif.sh, and scripts/prove-real-voice.sh. Capture the landing/demo assets from a real-provider run when maintainer keys are available, then check public/assets/screenshots/, docs/assets/screenshots/, public/landing/index.html, and the README. Done means the PNG/GIF, alt text, and mock-versus-real wording honestly match the captured UI.

Written by the indexing model from the issue text.