Dampen mobile realtime voice barge-in echo

[rogerchappel]

Summary

• use a stricter barge-in profile for mobile and Capacitor runtimes
• add a short output grace window and require sustained stronger speech before interrupting on mobile
• suppress likely speaker echo frames from being sent upstream on mobile until barge-in is confirmed

Verification

• pnpm test -- src/lib/realtime-voice-gateway-relay.test.ts src/lib/realtime-voice-client.test.ts src/lib/realtime-voice-audio.test.ts
• pnpm typecheck
• git diff --check
• pre-push pnpm typecheck && pnpm build

Risk

Medium. This changes realtime interrupt behavior, but only in the realtime gateway relay client. Desktop keeps its existing sensitivity; mobile/Capacitor gets echo damping.