compose: Shrink "send" button to match content-input height, like before M3

[chrisbobbe]

The way to get the smaller IconButton height we want has changed. After migrating to Material Design 3, in #380, we have to do this different thing.

Fixes: #398

[chrisbobbe]

Before	After

[gnprice]

Thanks for the fix!

Merging, after adjusting the comment to be more definitive:

         style: const ButtonStyle(
-          // Match the height of the content input. Empirically, setting
-          // [MaterialTapTargetSize.shrinkWrap] seems necessary to let the size
-          // be smaller than 48px square (at least without messing with
-          // [visualDensity]). So do that. It would be nice if the touch target
-          // extended invisibly out from the button, to make a 48px square,
-          // but I can't easily check that in the inspector. It might be the
-          // behavior, from reading docs, but the docs aren't clear enough
-          // to rely on.
+          // Match the height of the content input.
           minimumSize: MaterialStatePropertyAll(Size.square(_sendButtonSize)),
+          // With the default of [MaterialTapTargetSize.padded], not just the
+          // tap target but the visual button would get padded to 48px square.
+          // It would be nice if the tap target extended invisibly out from the
+          // button, to make a 48px square, but that's not the behavior we get.
           tapTargetSize: MaterialTapTargetSize.shrinkWrap,