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.

examples/gallery/pages/mod.rs

@@ -10,6 +10,7 @@ mod editor;
 mod feedback;
 mod finance;
 mod forms;
+mod generative;
 mod git;
 mod layout;
 mod lists;
@@ -63,6 +64,7 @@ pub const ALL: &[Page] = &[
     collab::PAGE,
     chat::PAGE,
     agent::PAGE,
+    generative::PAGE,
     theme::PAGE,
 ];