compose [nfc]: Refactor outer vertical padding on label in _Banner

Move vertical padding so it won't wrap the trailing element, when
present (as a `buildTrailing` that returns non-null). When we add
the "Cancel" and "Save" buttons for the edit-message banner, those
will want to control their own outer vertical padding to make it
respond to taps, because the painted button is supposed to be just
28px tall, which is too small for a touch target; see discussion
linked in dartdoc.

NFC because `buildTrailing` on _Banner's only subclass,
_ErrorBanner, returns null.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1400,7 +1400,13 @@ abstract class _Banner extends StatelessWidget {
   Color getLabelColor(DesignVariables designVariables);
   Color getBackgroundColor(DesignVariables designVariables);
 
-  /// A trailing element, with no outer padding for spacing/positioning.
+  /// A trailing element, with vertical but not horizontal outer padding
+  /// for spacing/positioning.
+  ///
+  /// An interactive element's touchable area should have height at least 44px,
+  /// with some of that as "slop" vertical outer padding above and below
+  /// what gets painted:
+  ///   https://github.com/zulip/zulip-flutter/pull/1432#discussion_r2023907300
   ///
   /// To control the element's distance from the end edge, override [padEnd].
   Widget? buildTrailing(BuildContext context);
@@ -1431,12 +1437,12 @@ abstract class _Banner extends StatelessWidget {
           // (SafeArea.minimum doesn't take an EdgeInsetsDirectional)
           .resolve(Directionality.of(context)),
         child: Padding(
-          padding: const EdgeInsetsDirectional.fromSTEB(8, 5, 0, 5),
+          padding: const EdgeInsetsDirectional.only(start: 8),
           child: Row(
             children: [
               Expanded(
                 child: Padding(
-                  padding: const EdgeInsets.symmetric(vertical: 4),
+                  padding: const EdgeInsets.symmetric(vertical: 9),
                   child: Text(
                     style: labelTextStyle,
                     textScaler: MediaQuery.textScalerOf(context).clamp(maxScaleFactor: 1.5),