dismiss the lightbox on a tap outside the image (T-309)

The image fills the 94% viewer with a transparent letterbox, so taps on
the dimmed canvas beside the image hit the viewer and did nothing — only
the 6% margin dismissed. Add onTapUp that dismisses when the tap lands
outside the actual painted-image rect (computed from the RenderImage +
applyBoxFit, transformed for any zoom/pan). Double-tap reset, scroll
zoom, pan, Esc, and the close button are unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -36,6 +36,11 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- **Clicking outside the image in the lightbox now closes it.** Previously only
+  the thin margin dismissed — a click on the dimmed canvas beside a letterboxed
+  image hit the viewer and did nothing. A single tap outside the painted image
+  now closes it (matching Esc / the × button); tapping, dragging, or zooming the
+  image still doesn't. (T-309)
 - **Run-status indicators no longer crash on rapid flips.** Switching status
   back and forth within the 200ms cross-fade (e.g. running → success → running
   across two bound Claude panes) tripped an AnimatedSwitcher duplicate-key