Iteration 5: add browser voice mode

[HimanM]

Summary

• add browser-native speech helpers for microphone input and spoken assistant replies
• add a mic button with listening state plus a spoken-reply toggle in the existing input bar
• keep the text chat input working when voice APIs are unavailable and document the verification in the checklist

Verification

• node --test --experimental-strip-types frontend/src/lib/speech.spec.ts
• npm run lint
• npm run build
• Playwright fallback against http://localhost:3000 with mocked supported and unsupported speech APIs