T26a Media: images

A new chapter, src/media. ImageViewer fits a picture to its stage and
zooms by the document steps, by keys, buttons and Cmd-scroll about the
pointer; a drag through gpui's drag, a scroll or the arrows pan it once it
overhangs; R turns it a quarter at a time. gpui paints pictures without a
transform, so media::turn turns the decoded frames off the main thread,
an asset keyed by the picture and the turn; `image` 0.25, which gpui
already decodes with, is now a dependency. ImageThumbnail shows a picture
in its own shape with a corner label and a ring when chosen, and is a Tab
stop only with a handler. Lightbox / Gallery Viewer points to
overlays::Lightbox, which the gallery opens from the thumbnails.

T26 splits into T26a–e. AGENTS.md: the drag rule says drags that pan,
turn or slide go through gpui's drag; pictures turn by their pixels.

src/documents/mod.rs

@@ -45,6 +45,7 @@ pub use templates::{Template, TemplatePicker};
 pub use thumbs::PageThumbnailList;
 pub use todo::{Checklist, TodoItem};
 pub use toolbar::FixedFormatToolbar;
+pub(crate) use viewer::step as zoom_step;
 pub use viewer::{DocPage, DocumentViewer, PageHit, PageNote};
 
 /// Binds the documents keys. `init` calls it.