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>

tasks/ch21-30.md

@@ -251,16 +251,26 @@
 - [x] PromptPlayground
 - [x] PromptDiff / PromptVersionHistory
 
-## T26 Media — `src/media`
+## T26a Media · Images — `src/media`
+
+- [x] ImageViewer — zoom, pan, rotate, fit
+- [x] ImageThumbnail
+- [x] Lightbox / Gallery Viewer → `overlays::Lightbox`
+
+## T26b Media · Crop and annotate
 
-- [ ] ImageViewer — zoom, pan, rotate, fit
 - [ ] ImageCropper — also T06e ImageUpload
 - [ ] ImageAnnotator
-- [ ] ImageThumbnail
-- [ ] Lightbox / Gallery Viewer → `overlays::Lightbox`
+
+## T26c Media · Video
+
 - [ ] VideoPlayer — host frames
 - [ ] VideoTimeline / Scrubber
 - [ ] VideoThumbnailStrip
+- [ ] SubtitleEditor
+
+## T26d Media · Audio and controls
+
 - [ ] AudioPlayer — host playback
 - [ ] AudioWaveform
 - [ ] AudioSpectrum / Visualizer
@@ -268,7 +278,9 @@
 - [ ] PlaybackSpeedControl
 - [ ] MediaControls
 - [ ] Playlist
-- [ ] SubtitleEditor
+
+## T26e Media · Capture and devices
+
 - [ ] ScreenRecorderControls
 - [ ] CameraPreview — host frames
 - [ ] MicLevelMeter