reactions [nfc]: Refactor text-emoji minimum height

This seems to do the job more transparently, and it does it without
needing to use the heard-coded value 2 and explain what it means.
But I've separated this from the previous commit because it's a less
minimal change from the status quo.

lib/widgets/emoji_reaction.dart

@@ -159,13 +159,15 @@ class ReactionChip extends StatelessWidget {
                   mainAxisSize: MainAxisSize.min,
                   crossAxisAlignment: CrossAxisAlignment.center,
                   children: [
-                    // So text-emoji chips are at least as tall as square-emoji
-                    // ones (probably a good thing).
-                    // 2 is for the `vertical: 1` padding around `emoji`.
-                    SizedBox(height: _squareEmojiScalerClamped(context).scale(_squareEmojiSize) + 2),
                     Flexible( // [Flexible] to let text emojis expand if they can
                       child: Padding(padding: const EdgeInsets.symmetric(horizontal: 3, vertical: 1),
-                        child: emoji)),
+                        child: ConstrainedBox(
+                          constraints: BoxConstraints(
+                            // minHeight so text-emoji chips are at least as tall
+                            // as square-emoji ones (probably a good thing).
+                            minHeight: _squareEmojiScalerClamped(context).scale(_squareEmojiSize)),
+                          child: Center(widthFactor: 1,
+                            child: emoji)))),
                     // Added vertical: 1 to give some space when the label is
                     // taller than the emoji (e.g. because it needs multiple lines)
                     Padding(padding: const EdgeInsets.symmetric(horizontal: 3, vertical: 1),