Refresh landing/demo screenshots and GIF after a real-provider run #50

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.