- Single 'Play narration' button: checks presign first, plays immediately if audio exists, otherwise triggers generation
- During generation shows an animated pseudo progress bar: slow start (4%/s) → accelerates (12%/s at 30%) → slows near 80% (4%/s) → crawls to 99% (0.3%/s)
- When generation completes, bar jumps to 100% then transitions to audio player
- Auto-plays audio after generation completes
