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.

examples/gallery/pages/generative/mod.rs

@@ -1,6 +1,7 @@
 mod canvas;
 mod panel;
 mod results;
+mod sound;
 
 use gpui::{AnyElement, App, IntoElement, ParentElement, Window, div};
 
@@ -43,5 +44,8 @@ fn render(window: &mut Window, cx: &mut App) -> AnyElement {
         .child(results::results(window, cx))
         .child(results::variations(window, cx))
         .child(canvas::canvas(window, cx))
+        .child(sound::audio(window, cx))
+        .child(sound::voice(window, cx))
+        .child(sound::video(window, cx))
         .into_any_element()
 }