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.

src/theme/generative.rs

@@ -7,12 +7,18 @@ use super::{Theme, tokens::px_to_rems};
 pub struct GenerativeSizes {
     /// A style preset's picture, square.
     pub preset: Rems,
+    /// A second of video along a timeline.
+    pub second: Rems,
+    /// A shot's height on a timeline.
+    pub shot: Rems,
 }
 
 impl Theme {
     pub fn generative(&self) -> GenerativeSizes {
         GenerativeSizes {
             preset: px_to_rems(88.0),
+            second: px_to_rems(24.0),
+            shot: px_to_rems(56.0),
         }
     }
 }