Handle browser AudioContext suspension by resuming on use, pass ctx
parameter to playTone to avoid redundant getAudioContext calls, and
wrap all play functions in try/catch for graceful degradation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Synthesize short tones via Web Audio API - no external audio files needed.
Correct answers play an ascending two-note ding, incorrect answers play a
gentle descending tone, and lesson completion plays a four-note fanfare.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Add /api/tts endpoint using msedge-tts crate for high-quality
Filipino neural voices (fil-PH-BlessicaNeural default)
- Disk-based LRU cache in data/tts-cache/ to avoid re-synthesis
- Update useSpeech hook to fetch from server and play audio via
HTMLAudioElement instead of Web Speech API
- Add loading spinner to SpeakButton while audio is fetched
- TTS is now always available regardless of browser support

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>