compose: In error banner, use SafeArea.minimum for trailing padding

Before, the 8px end padding would just be added to any SafeArea
padding. Now, it's done using SafeArea.minimum, so it doesn't get
too big.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1333,7 +1333,7 @@ abstract class _Banner extends StatelessWidget {
   /// To control the element's distance from the end edge, override [padEnd].
   Widget? buildTrailing(BuildContext context);
 
-  /// Whether to add 8px trailing padding.
+  /// Whether to apply `end: 8` in [SafeArea.minimum].
   ///
   /// Subclasses can use `false` when the [buildTrailing] element
   /// is meant to abut the edge of the screen
@@ -1354,7 +1354,7 @@ abstract class _Banner extends StatelessWidget {
       decoration: BoxDecoration(
         color: getBackgroundColor(designVariables)),
       child: SafeArea(
-        minimum: const EdgeInsetsDirectional.only(start: 8)
+        minimum: EdgeInsetsDirectional.only(start: 8, end: padEnd ? 8 : 0)
           // (SafeArea.minimum doesn't take an EdgeInsetsDirectional)
           .resolve(Directionality.of(context)),
         child: Padding(
@@ -1370,7 +1370,6 @@ abstract class _Banner extends StatelessWidget {
                 const SizedBox(width: 8),
                 trailing,
               ],
-              if (padEnd) const SizedBox(width: 8),
             ]))));
   }
 }