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>

test/widgets/clide_lightbox_test.dart

@@ -60,6 +60,32 @@ void main() {
     expect(find.byType(InteractiveViewer), findsOneWidget);
   });
 
+  testWidgets('a tap on the dimmed canvas dismisses; a tap on the image does not (T-309)', (tester) async {
+    final img = (await tester.runAsync(() => createTestImage(width: 100, height: 100)))!; // 1:1; real async
+    var dismissed = false;
+    await tester.pumpWidget(harness(
+      f,
+      MediaQuery(
+        data: const MediaQueryData(size: Size(800, 600)),
+        child: ClideLightbox(onDismiss: () => dismissed = true, child: RawImage(image: img, fit: BoxFit.contain)),
+      ),
+    ));
+    await tester.pumpAndSettle();
+
+    // The 94% box is ~752×564; a 1:1 image fits to 564×564 centred, leaving
+    // ~94px side margins. A tap in the left margin is dimmed canvas → dismiss.
+    final r = tester.getRect(find.byType(InteractiveViewer));
+    await tester.tapAt(Offset(r.left + 10, r.center.dy));
+    await tester.pump(const Duration(milliseconds: 350)); // past the double-tap delay
+    expect(dismissed, isTrue);
+
+    // A tap on the image itself does not dismiss.
+    dismissed = false;
+    await tester.tapAt(r.center);
+    await tester.pump(const Duration(milliseconds: 350));
+    expect(dismissed, isFalse);
+  });
+
   testWidgets('scroll wheel zooms in and out', (tester) async {
     await tester.pumpWidget(harness(f, box(() {})));
     await tester.pumpAndSettle();