fix: use browser speechSynthesis for playback when browser-native-tts is selected

[YizukiAme]

Summary

Fix browser-native TTS producing no sound during classroom playback, while the settings test plays sound correctly.

Fixes #25, fixes #12, fixes #5

Root Cause

When browser-native-tts is selected as the TTS provider:

1. Scene generation (use-scene-generator.ts:214,450) correctly skips pre-generating audio — browser TTS runs client-side via Web Speech API, not via server API
2. Playback (engine.ts:436-444) calls audioPlayer.play() which finds no pre-generated audio in IndexedDB → returns false → falls back to scheduleReadingTimer() — a silent timer that estimates reading time but never calls speechSynthesis

Fix

Add Web Speech API integration directly in PlaybackEngine (lib/playback/engine.ts):

• playBrowserTTS() — speaks text via window.speechSynthesis, respecting user's voice, speed, volume, and mute settings
• cancelBrowserTTS() — cancels active browser TTS
• pause() — calls speechSynthesis.pause() when browser TTS is active
• resume() — calls speechSynthesis.resume() when browser TTS is paused
• stop() / handleUserInterrupt() — calls speechSynthesis.cancel() to stop browser TTS

The fix is self-contained in one file. When audioPlayer.play() returns false (no pre-generated audio), the engine now checks if browser-native-tts is the selected provider and calls speechSynthesis.speak() instead of falling back to the silent reading timer.

Changes

File	Change
lib/playback/engine.ts	+83 lines, -4 lines

Testing

• Set TTS Provider to "Browser Native" → settings test plays sound ✅
• Generate classroom → play → sound plays ✅
• Pause/resume works correctly
• No regression for other TTS providers (they still use pre-generated audio path)

[cosarah]

Code Review

Clean, focused change with correct edge case handling. Overall LGTM. Two items to consider as follow-up improvements:

1. Chrome long utterance cutoff

Chrome has a known bug where SpeechSynthesisUtterance longer than ~15 seconds gets silently cut off. If a speechAction.text is long, playback may stop mid-sentence and onend won't fire, causing the playback engine to hang. A follow-up PR could add a workaround (e.g., chunking text or adding a timeout watchdog).

2. speechSynthesis.pause()/resume() cross-browser compatibility

Firefox has incomplete support for speechSynthesis.pause() / speechSynthesis.resume(), which may cause playback to not recover after pausing. This is a Web Speech API platform limitation and doesn't affect the correctness of this PR, but worth noting.

[YizukiAme]

Thanks for the review!!!! I'll address the Chrome 15s cutoff and Firefox pause/resume issues in a follow-up PR by implementing an utterance queue with text chunking. This will elegantly handle both issues while keeping this PR focused on the basic fallback.