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/primitives/icon.rs

@@ -56,6 +56,7 @@ icons! {
     ArrowUpRight => "arrow-up-right",
     ArrowUp => "arrow-up",
     AtSign => "at-sign",
+    AudioLines => "audio-lines",
     Ban => "ban",
     Battery => "battery",
     Bell => "bell",