fix(tsconnect/wasm): nil-check lb and ln in shutdown() before use

lb and ln are only initialised during run(); calling shutdown() before
run() panics on nil. Guard both fields before dereferencing.

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

cmd/tsconnect/wasm/wasm_js.go

@@ -637,8 +637,12 @@ func (i *jsIPN) logout() {
 func (i *jsIPN) shutdown() js.Value {
 	return makePromise(func() (any, error) {
 		i.shutdownOnce.Do(func() {
-			i.lb.Shutdown()
-			i.ln.Close()
+			if i.lb != nil {
+				i.lb.Shutdown()
+			}
+			if i.ln != nil {
+				i.ln.Close()
+			}
 			close(i.shutdownCh)
 		})
 		return nil, nil