fix(config): allow digits in config keys so keys like gr_1hop round-trip

src/helpers/ConfigSerializer.cpp

@@ -24,7 +24,9 @@ static bool is_whitespace(char c) {
   return c == ' ' || c == '\t' || c == '\r' || c == '\n';
 }
 static bool is_key_char(char c) {
-  return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == '_';
+  // digits are allowed: keys like gr_1hop contain one, otherwise loadSerial
+  // fails at that key and prefs stop reloading after a reboot
+  return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9') || c == '_';
 }
 static bool is_value_char(char c) {
   return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || c == '-' || c == '.';