refactor: Convert collapser span to button for accessibility

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

webclient/src/Comments.js

@@ -85,7 +85,7 @@ function Comments({ cache }) {
 						{' '} | <HashLink to={'#'+cid} id={cid}>{moment.unix(c.date).fromNow()}</HashLink>
 
 						{hidden || hasChildren &&
-							<span className='collapser pointer' onClick={() => collapseComment(cid)}>–</span>
+							<button className='collapser pointer' onClick={() => collapseComment(cid)}>–</button>
 						}
 					</p>
 				</div>

webclient/src/Style-light.css

@@ -215,6 +215,16 @@ span.source {
 	padding-right: 1.5rem;
 }
 
+button.collapser {
+	background: transparent;
+	border: none;
+	margin: 0;
+	padding-top: 0;
+	padding-bottom: 0;
+	font: inherit;
+	color: inherit;
+}
+
 .comment .pointer {
 	cursor: pointer;
 }