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

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

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -761,12 +761,18 @@ class _SendButtonState extends State<_SendButton> {
       ),
       child: IconButton(
         tooltip: zulipLocalizations.composeBoxSendTooltip,
-
-        // Match the height of the content input. Zeroing the padding lets the
-        // constraints take over.
-        constraints: const BoxConstraints(minWidth: _sendButtonSize, minHeight: _sendButtonSize),
-        padding: const EdgeInsets.all(0),
-
+        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.
+          minimumSize: MaterialStatePropertyAll(Size.square(_sendButtonSize)),
+          tapTargetSize: MaterialTapTargetSize.shrinkWrap,
+        ),
         color: foregroundColor,
         icon: const Icon(Icons.send),
         onPressed: _send));