Label realtime-capable voice choices

[rogerchappel]

Summary

• add a Realtime filter to the voice selector
• badge voice options as Realtime or TTS based on the OpenAI realtime allowlist
• share the realtime voice allowlist between selector display and realtime session startup

Verification

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

Risk

Low/medium: UI labeling and filter behavior only, with the runtime allowlist moved into shared voice metadata.