fix(wasm): correct ICMP case in ping type error message

The constant tailcfg.PingICMP is "ICMP" not "icmp"; the error message
was listing the wrong string, causing user confusion about valid values.

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

cmd/tsconnect/wasm/wasm_js.go

@@ -1278,7 +1278,7 @@ func (i *jsIPN) ping(ip string, pingType string, size int) js.Value {
 		case tailcfg.PingDisco, tailcfg.PingTSMP, tailcfg.PingICMP, tailcfg.PingPeerAPI:
 			// valid
 		default:
-			return nil, fmt.Errorf("ping: unknown type %q, must be one of: disco, TSMP, icmp, peerapi", pingType)
+			return nil, fmt.Errorf("ping: unknown type %q, must be one of: disco, TSMP, ICMP, peerapi", pingType)
 		}
 		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 		defer cancel()