fix branch picker: escape to dismiss, error display, backdrop click

Branch picker now handles Escape key to dismiss, shows an × close
button in the header, and displays error/empty states instead of
silently showing nothing. Dialog backdrop is click-to-dismiss (the
dialog content stops propagation so clicks inside don't close it).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/dialog.dart

@@ -93,10 +93,16 @@ class DialogHost extends StatelessWidget {
             final b = router.current;
             if (b == null) return const SizedBox.shrink();
             return Positioned.fill(
-              child: ColoredBox(
-                color: backdropColor,
-                child: Center(
-                  child: b(ctx, router.dismiss),
+              child: GestureDetector(
+                onTap: () => router.dismiss(),
+                child: ColoredBox(
+                  color: backdropColor,
+                  child: Center(
+                    child: GestureDetector(
+                      onTap: () {},
+                      child: b(ctx, router.dismiss),
+                    ),
+                  ),
                 ),
               ),
             );