feat(web): clear-log button on the event log panel

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

web/src/pages/TaskDetail.tsx

@@ -291,6 +291,11 @@ export function TaskDetail({ id }: { id: number }) {
 
         <div className="panel">
           <span className="panel-label">Event log</span>
+          {log.length > 0 && (
+            <button type="button" className="link-btn log-clear" onClick={() => setLog([])}>
+              clear log
+            </button>
+          )}
           <div className="log-pane">
             {log.length === 0 ? (
               <div className="log-empty">awaiting events over websocket…</div>