fix: make sleep stats optional to prevent page crash

If stats endpoint fails, page will still load with empty stats.
This prevents 500 errors from blocking the entire sleep page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/SleepPage.jsx

@@ -44,7 +44,7 @@ export default function SleepPage() {
     setLoading(true)
     Promise.all([
       api.listSleep(30),
-      api.getSleepStats(7)
+      api.getSleepStats(7).catch(() => null) // Stats optional - don't fail if error
     ]).then(([sleepData, statsData]) => {
       setSleep(sleepData)
       setStats(statsData)