T25d Generative: sound and motion

AudioGenerationPlayer shows a generated sound's prompt, model and
length, a bar while it is made, then chat::AudioMessage to play it;
Regenerate and Download wait for it. TTSVoicePicker lists voices with a
sample to play, the chosen one washed with a check, tags dropping below
when narrow. VideoGenerationTimeline lays shots along their time under
a ruler, with posters, placeholders, a playhead and Extend.

AudioMessage's waveform is one Tab stop where Left and Right seek, and
it gives way at 280px. debug::ruler serves the timeline's ruler. A mask
shown to read no longer follows the pointer.

tasks/ch21-30.md

@@ -232,9 +232,9 @@
 
 ## T25d Generative · Sound and motion
 
-- [ ] AudioGenerationPlayer
-- [ ] VideoGenerationTimeline
-- [ ] TTSVoicePicker
+- [x] AudioGenerationPlayer
+- [x] VideoGenerationTimeline
+- [x] TTSVoicePicker
 
 ## T25e Generative · Models