fix: honor explicit TTS voice selections

[rogerchappel]

Summary

• route explicit OpenAI/ElevenLabs voice selections through server TTS even when native preference is set
• avoid silently falling back to browser/native/default voices for explicit server voice selections
• return a 503 from /api/tts when a selected server voice provider is unavailable instead of masking it with local TTS

Verification

• pnpm typecheck
• git diff --check
• push hook: pnpm typecheck && pnpm build

Risk

Medium. Touches voice playback routing and TTS fallback behavior; explicit server voice failures will now stop instead of using the default voice.