fix: tune iOS native voice capture

[rogerchappel]

Summary

• lengthen native iOS VAD start/end timing so natural pauses do not submit early
• add an adaptive native noise floor to reduce AirPods false starts
• explicitly prefer available voice inputs, falling back to the built-in mic when no headset mic is present
• discard low-content native transcripts like punctuation-only output before they create chat messages

Verification

• node --check apps/mobile/scripts/ensure-ios-voice-session.mjs
• node apps/mobile/scripts/ensure-ios-voice-session.mjs
• git diff --check HEAD~1..HEAD
• pnpm test src/lib/native-voice-session-generator.test.ts
• pnpm typecheck
• push hook: pnpm typecheck && pnpm build

Risk

Medium. Touches iOS native audio capture routing and VAD timing; needs device QA with AirPods and built-in iPhone mic.