zero pad decimal part of average guesses

An average of 3.04 was shown as 3.4 because the hundredths weren't
padded. Pad with parameter expansion to keep the script bash-only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

wordle

@@ -484,6 +484,7 @@ function update_stats() {
     integer_part=$(( result / 100 ))
     decimal_part=$(( result % 100 ))
 
+    [[ ${decimal_part} -lt 10 ]] && decimal_part="0${decimal_part}"
     AVERAGE="${integer_part}.${decimal_part}"
   fi