fix: Widen grp data type

ref: #1928

src/helpers/TxtDataHelpers.h

@@ -3,10 +3,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#define TXT_TYPE_PLAIN          0    // a plain text message
-#define TXT_TYPE_CLI_DATA       1    // a CLI command
-#define TXT_TYPE_SIGNED_PLAIN   2    // plain text, signed by sender
-#define DATA_TYPE_CUSTOM        0xFF // custom app binary payload (group/channel datagrams)
+#define TXT_TYPE_PLAIN          0      // a plain text message
+#define TXT_TYPE_CLI_DATA       1      // a CLI command
+#define TXT_TYPE_SIGNED_PLAIN   2      // plain text, signed by sender
+#define DATA_TYPE_CUSTOM        0xFFFF // generic custom app namespace for group/channel datagrams
 
 class StrHelper {
 public: