make output row flexbox so operator buttons don't get huge on zoom

index.html

@@ -10,10 +10,9 @@
 			<label for="expr">Expression</label>
 			<input id="expr" required type="text" />
 
-			<output id="result" for="expr modulus"></output>
-
-			<div id="modulus-entry">
-				<label for="modulus"> mod </label>
+			<div id="result-row">
+				<output id="result" for="expr modulus"></output>
+				<label for="modulus">mod</label>
 				<input id="modulus" required type="number" min="1" step="1" />
 			</div>