fix(ui): preserve environment selection across logout

handleLogout explicitly cleared the env from localStorage, forcing the
env switcher modal to re-open on every login. Drop that clear so the
last selected env is restored from localStorage on the next session —
the expected behavior for a personal-preference store.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/components/LayoutShell.tsx

@@ -729,7 +729,6 @@ function LayoutContent() {
   // --- Callbacks ----------------------------------------------------
   const handleLogout = useCallback(() => {
     logout();
-    useEnvironmentStore.getState().setEnvironment(undefined);
     navigate('/login');
   }, [logout, navigate]);