fix(shell): clear stale taskbar entries on WebSocket reconnect

When weft-appd restarts, session IDs reset to 0. Without clearing,
RUNNING_APPS repopulation after reconnect would coexist with stale
entries from the previous session. Removing all weft-taskbar-app
elements on the open event ensures the taskbar reflects only the
current appd session registry.

infra/shell/system-ui.html

@@ -256,6 +256,7 @@
 
         ws.addEventListener('open', function () {
           wsReconnectDelay = 1000;
+          document.querySelectorAll('weft-taskbar-app').forEach(function (el) { el.remove(); });
           ws.send(JSON.stringify({ type: 'QUERY_RUNNING' }));
           ws.send(JSON.stringify({ type: 'QUERY_INSTALLED_APPS' }));
         });