fix(blind-mode): make blind mode "on" text invisible (@Leonabcd123) (monkeytypegame#7274)

### Description

Use ` ` instead of the current, visible character.

Author: Leonabcd123

frontend/src/html/pages/settings.html

@@ -192,7 +192,7 @@
       <div class="buttons">
         <button data-config-value="false">off</button>
         <button data-config-value="true">
-          ⠀
+          &ensp;
           <!-- On is missing on purpose. -->
         </button>
       </div>