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.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -20,6 +20,7 @@ barcoders = { version = "2", default-features = false, features = ["std"] }
 emojis = "0.9"
 futures = "0.3"
 gpui = "0.2.2"
+image = { version = "0.25", default-features = false }
 iso_currency = { version = "0.7", features = ["iterator"] }
 isocountry = "0.3"
 isolang = { version = "2.4", features = ["local_names", "list_languages"] }