Fix Invalid Date for datetime strings from API

The API returns dates as "2024-12-11T00:00:00" (datetime, no timezone),
not bare "2024-12-11". Appending T12:00:00 directly produced an invalid
string. Use .slice(0, 10) to extract the date part first before parsing.

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

web/src/pages/SessionPage.tsx

@@ -24,7 +24,7 @@ interface SessionDetail {
 }
 
 function formatDate(iso: string): string {
-  const d = new Date(iso + "T12:00:00");
+  const d = new Date(iso.slice(0, 10) + "T12:00:00");
   return d.toLocaleDateString(undefined, { weekday: "long", year: "numeric", month: "long", day: "numeric" });
 }