T25a Generative: prompt and settings

PromptEnhancer offers a fuller prompt with the words it adds washed,
split at punctuation; Use this takes it into the field, Keep mine
lets it go. StylePresetPicker and AspectRatioPicker are rows of Tab
stops, the chosen one in accent; a ratio chip draws its shape.
SeedInput keeps digits up to u32::MAX and rolls one with a die.
NegativePromptInput and ImageGenerationPanel point to forms::TagInput
and forms::Form. theme.generative() holds the chapter's sizes. The
gallery's preset pictures are the atrium photo in five styles.

src/theme/mod.rs

@@ -1,4 +1,5 @@
 mod chart;
+mod generative;
 mod palette;
 mod tokens;
 
@@ -8,6 +9,7 @@ use gpui::{App, Global, SharedString, WindowAppearance};
 
 use crate::motion;
 pub use chart::ChartSizes;
+pub use generative::GenerativeSizes;
 pub use palette::{Mix, Palette, Syntax};
 pub use tokens::{
     AvatarSize, ContainerSize, ControlSize, Density, Elevation, IconSize, Radius, TextSize,