fix(stats): address Claude review follow-ups

stats/src/hooks/useSessions.test.ts

@@ -0,0 +1,20 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import test from 'node:test';
+import { fileURLToPath } from 'node:url';
+import { toErrorMessage } from './useSessions';
+
+const USE_SESSIONS_PATH = fileURLToPath(new URL('./useSessions.ts', import.meta.url));
+
+test('toErrorMessage normalizes Error and non-Error rejections', () => {
+  assert.equal(toErrorMessage(new Error('network down')), 'network down');
+  assert.equal(toErrorMessage('bad gateway'), 'bad gateway');
+  assert.equal(toErrorMessage(503), '503');
+});
+
+test('useSessions and useSessionDetail route catch handlers through toErrorMessage', () => {
+  const source = fs.readFileSync(USE_SESSIONS_PATH, 'utf8');
+  const matches = source.match(/setError\(toErrorMessage\(err\)\)/g);
+
+  assert.equal(matches?.length, 2);
+});

stats/src/hooks/useSessions.ts

@@ -3,6 +3,10 @@ import { getStatsClient } from './useStatsApi';
 import { SESSION_CHART_EVENT_TYPES } from '../lib/session-events';
 import type { SessionSummary, SessionTimelinePoint, SessionEvent } from '../types/stats';
 
+export function toErrorMessage(err: unknown): string {
+  return err instanceof Error ? err.message : String(err);
+}
+
 export function useSessions(limit = 50) {
   const [sessions, setSessions] = useState<SessionSummary[]>([]);
   const [loading, setLoading] = useState(true);
@@ -21,7 +25,7 @@ export function useSessions(limit = 50) {
       })
       .catch((err) => {
         if (cancelled) return;
-        setError(err.message);
+        setError(toErrorMessage(err));
       })
       .finally(() => {
         if (cancelled) return;
@@ -77,7 +81,7 @@ export function useSessionDetail(sessionId: number | null) {
       })
       .catch((err) => {
         if (cancelled) return;
-        setError(err.message);
+        setError(toErrorMessage(err));
       })
       .finally(() => {
         if (cancelled) return;