fix(ui): open one session accordion panel at a time

frontend/src/shell/AppShell.tsx

@@ -1160,7 +1160,7 @@ export function AppShell({ canLogout }: AppShellProps) {
             </p>
           )}
 
-          <Accordion.Root multiple
+          <Accordion.Root multiple={false}
             value={[...(activeOpen ? ["active"] : []), ...(archiveOpen ? ["archive"] : [])]}
             onValueChange={value => { setActiveOpen(value.includes("active")); setArchiveOpen(value.includes("archive")); }}
             className="thot-session-accordion min-h-0 flex-1 overflow-y-auto px-2 pb-4">
@@ -1210,8 +1210,7 @@ export function AppShell({ canLogout }: AppShellProps) {
                 </div>
               ))}
               {/* Ungrouped sessions list directly after the groups — no "No group" label.
-                  When there are no groups at all, still render it so its empty state can
-                  teach first-time users. */}
+                  When there are no groups at all, still render its empty state. */}
               {(ungroupedActive.length > 0 || groups.length === 0) && (
                 <NavSessions
                   sessions={ungroupedActive}