Show warning when custom username and cloud variables are used at the same time

src/locales/en.json

@@ -157,6 +157,10 @@
     "username": {
       "string": "Username (each \"#\" gets replaced with a random number)"
     },
+    "customUsernameWarning": {
+      "string": "Be careful when changing the default username as it can cause connections to the default cloud variable server to be blocked. Only the default username is guaranteed to work.",
+      "context": "Warning that appears when using custom username and cloud variables at the same time."
+    },
     "autoplay": {
       "string": "Start project automatically instead of showing a big green flag"
     },

src/p4/PackagerOptions.svelte

@@ -295,6 +295,11 @@
       {$_('options.username')}
       <input type="text" class="shorter" bind:value={$options.username}>
     </label>
+    {#if $options.username !== defaultOptions.username && cloudVariables.length !== 0}
+      <p class="warning">
+        {$_('options.customUsernameWarning')}
+      </p>
+    {/if}
 
     <h3>{$_('options.stage')}</h3>
     <label class="option">