feat: guided light meditation + Edge TTS neural voice audio

- Add continuous guided light-body-scan meditation (16 segments, 97% coverage)
- Edge TTS API endpoint with Xiaoxiao neural voice at -40% rate
- Pre-generated chime WAVs and guidance MP3s in public/audio/
- Mute toggle in practice page, sequenced non-overlapping audio
- Fix middleware to serve /audio and /api/audio/tts without auth
- All LLM output localized to Chinese; fix blank home/settings pages
- DeepSeek API compat: json_object, increased max_tokens
- Analytics, cron reminders, onboarding flow fixes

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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