feat: integrate Supertonic TTS with sentence-level chunk streaming

fix gradio version 5.33 for compatibility

fix bug of image path traversal

feat: full story text display with paragraph highlight on slider

feat: merge Library + Player into single tab with click-to-play

feat: replace Unsplash cover URLs with local assets/covers/ PNG paths

Load book list dynamically from stories/ folder

Add story dropdown + Play/Pause/Resume controls to Player tab (sprint #8)

Extract inline CSS to static/style.css + add mobile responsive rules

Add Gradio frontend: 5-tab UI with full demo loop screens