Voiceover generation progress bar

Generating a long voiceover used to show only a spinner, which made it feel like the app had frozen. You'll now see an animated progress bar and percentage under the "Generating voiceover…" badge, estimated from script length. It jumps to 100% as soon as the audio clip lands on the Audio track.

Real server-side streaming so you can start listening while Maya is still generating is tracked separately — see issue #11.