validate username in online mode

Usernames are used in file names and URLs, so a '/' could write files
outside the game directory and a space broke the share links. Only
allow a-z, 0-9 and _ (max 32 chars) and re-prompt otherwise.

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

wordle

@@ -140,9 +140,15 @@ function set_options() {
         echo -en "${CLEAR_SCREEN}"
 
         declare -g USER
-        read -r -p "Username: " USER
+        read -r -p "Username: " USER || exit 1
         USER="${USER,,}"
 
+        # Username is used in file names and URLs so only allow safe characters
+        while [[ -z "${USER}" || "${USER}" == *[!a-z0-9_]* || ${#USER} -gt 32 ]]; do
+          read -r -p "${CLEAR_UP1}Username (letters, numbers and _ only): " USER || exit 1
+          USER="${USER,,}"
+        done
+
         declare -g DAY
         DAY=$(( EPOCHSECONDS / 86400 ))