compose [nfc]: Refactor _Banner label's vertical padding

In the Figma:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4010-6425&m=dev
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4031-17028&m=dev
the banner label's 9px vertical padding is factored as 5px applied
to the whole banner, and 4px applied to just the label, excluding
trailing buttons. Follow that, so it can work as intended when we
add such buttons.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1400,17 +1400,19 @@ abstract class _Banner extends StatelessWidget {
         minimum: const EdgeInsetsDirectional.only(start: 8)
           // (SafeArea.minimum doesn't take an EdgeInsetsDirectional)
           .resolve(Directionality.of(context)),
-        child: Row(
-          children: [
-            Expanded(
-              child: Padding(
-                padding: const EdgeInsetsDirectional.fromSTEB(8, 9, 0, 9),
-                child: Text(style: labelTextStyle,
-                  label))),
-            const SizedBox(width: 8),
-            // TODO(#720) "x" button for the error banner goes here.
-            //   24px square with 8px touchable padding in all directions?
-          ])));
+        child: Padding(
+          padding: const EdgeInsetsDirectional.fromSTEB(8, 5, 0, 5),
+          child: Row(
+            children: [
+              Expanded(
+                child: Padding(
+                  padding: const EdgeInsets.symmetric(vertical: 4),
+                  child: Text(style: labelTextStyle,
+                    label))),
+              const SizedBox(width: 8),
+              // TODO(#720) "x" button for the error banner goes here.
+              //   24px square with 8px touchable padding in all directions?
+            ]))));
   }
 }