Default JSON log-line viewer nodes to collapsed

A log line whose text parses as JSON was rendering every nested node
fully expanded, dumping large payloads into the timeline instead of a
collapsed summary the user opts into expanding.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

webapp/src/components/LogPreview.tsx

@@ -113,7 +113,7 @@ function JsonNode({
   entries: ReadonlyArray<readonly [string, unknown]>
   bracket: [string, string]
 }) {
-  const [expanded, setExpanded] = useState(true)
+  const [expanded, setExpanded] = useState(false)
 
   if (entries.length === 0) {
     return (