Use cursor pointer for buttons

I'm usually not a fan of changing the default browser styles for
things like the cursor image, but here it actually improves
accessibility: it's clearer for the user what can be clicked on.

Closes: https://todo.sr.ht/~emersion/gamja/81

style.css

@@ -67,6 +67,7 @@ button {
 	border-color: var(--button-border);
 	border-width: 1px;
 	font-size: 0.8rem;
+	cursor: pointer;
 }
 
 button:hover {
@@ -351,6 +352,7 @@ a.nick:hover, a.nick:active {
 
 details summary[role="button"] {
 	user-select: none;
+	cursor: pointer;
 }
 
 #buffer {