Render JSON log lines as collapsible key/value trees (ticket 014)

LogPreview.tsx now detects when a log line's raw text parses as a JSON
object and renders it as a collapsible tree instead of raw text; any
other line (non-JSON or invalid JSON) keeps today's plain-text
rendering unchanged. Client-side only, no backend changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/LogPreview.tsx

@@ -82,6 +82,93 @@ function ColumnPicker({
   )
 }
 
+/// Attempts to parse a log line's raw text as a JSON object (not array/primitive), for structured
+/// tree rendering. Returns null for anything that isn't valid JSON or doesn't parse to a plain object,
+/// so those lines keep rendering as plain text.
+function tryParseJsonObject(text: string): Record<string, unknown> | null {
+  const trimmed = text.trim()
+  if (!trimmed.startsWith('{')) return null
+  try {
+    const parsed = JSON.parse(trimmed)
+    if (parsed !== null && typeof parsed === 'object' && !Array.isArray(parsed)) {
+      return parsed as Record<string, unknown>
+    }
+    return null
+  } catch {
+    return null
+  }
+}
+
+/// Renders a single JSON value: primitives inline, objects/arrays as collapsible subtrees.
+function JsonValueView({ value }: { value: unknown }) {
+  if (value === null) return <span className="text-text-subtle">null</span>
+  if (Array.isArray(value)) {
+    return <JsonNode entries={value.map((v, i) => [String(i), v] as const)} bracket={['[', ']']} />
+  }
+  if (typeof value === 'object') {
+    return <JsonNode entries={Object.entries(value as Record<string, unknown>)} bracket={['{', '}']} />
+  }
+  if (typeof value === 'string') return <span className="text-text">"{value}"</span>
+  return <span className="text-text-muted">{String(value)}</span>
+}
+
+function JsonNode({
+  entries,
+  bracket,
+}: {
+  entries: ReadonlyArray<readonly [string, unknown]>
+  bracket: [string, string]
+}) {
+  const [expanded, setExpanded] = useState(true)
+
+  if (entries.length === 0) {
+    return (
+      <span className="text-text-subtle">
+        {bracket[0]}
+        {bracket[1]}
+      </span>
+    )
+  }
+
+  return (
+    <span>
+      <button
+        type="button"
+        onClick={() => setExpanded((v) => !v)}
+        className="mr-1 inline-block w-3 select-none text-text-subtle hover:text-text-muted"
+        aria-label={expanded ? 'Collapse' : 'Expand'}
+      >
+        {expanded ? '▾' : '▸'}
+      </button>
+      {expanded ? (
+        <span>
+          {bracket[0]}
+          <span className="ml-3 block border-l border-border-faint pl-2">
+            {entries.map(([key, value]) => (
+              <span key={key} className="block">
+                <span className="text-text-secondary">{key}</span>
+                <span className="text-text-subtle">: </span>
+                <JsonValueView value={value} />
+              </span>
+            ))}
+          </span>
+          {bracket[1]}
+        </span>
+      ) : (
+        <span className="text-text-subtle">{bracket[0]}…{bracket[1]}</span>
+      )}
+    </span>
+  )
+}
+
+/// Renders a log line's message: a collapsible key/value tree when the raw text parses as a JSON
+/// object, otherwise the raw text unchanged.
+function LogMessage({ text }: { text: string }) {
+  const parsed = tryParseJsonObject(text)
+  if (!parsed) return <>{text}</>
+  return <JsonValueView value={parsed} />
+}
+
 /// Keeps the scroll position anchored to the same visible row across a `logs` update (e.g. a sliding time window
 /// pruning old rows from the front), instead of the browser's default of holding `scrollTop` constant in pixels —
 /// which would otherwise scroll the view to different content whenever rows are removed above it.
@@ -181,7 +268,9 @@ export function LogPreview({ logs, params }: { logs: LogEntry[]; params: ParamDe
                     </td>
                   )
                 })}
-                <td className="px-3 py-1 text-text">{log.text}</td>
+                <td className="px-3 py-1 text-text">
+                  <LogMessage text={log.text} />
+                </td>
               </tr>
             ))}
             {logs.length === 0 && (