Soften realtime voice barge-in sensitivity

[rogerchappel]

Summary

• Make web realtime barge-in require slightly stronger, sustained speech before cancelling playback
• Make mobile/Capacitor barge-in more conservative with stronger thresholds, more speech frames, and a longer output grace window
• Add regression coverage so short mobile speech bursts do not interrupt agent output

Verification

• pnpm test -- src/lib/realtime-voice-gateway-relay.test.ts
• pnpm typecheck
• git diff --check
• push hook: pnpm typecheck
• push hook: pnpm build (passes with existing Turbopack NFT-list warning)

Risk

Low. Scoped to realtime voice barge-in threshold constants and unit tests. The tradeoff is that intentional interruption now needs to be a bit more deliberate, especially on mobile.