Set temporary placeholder for input area.

Co-authored-by: Juliusz Chroboczek <jch@irif.fr>

static/galene.css

@@ -328,6 +328,10 @@ textarea.form-reply {
     transition: border-color ease-in-out .15s,box-shadow ease-in-out .15s;
 }
 
+.form-reply::placeholder {
+  opacity: .7;
+}
+
 .select {
     display: block;
     width: 100%;

static/galene.js

@@ -1481,6 +1481,11 @@ async function gotJoined(kind, group, perms, message) {
     displayUsername();
     setButtonsVisibility();
 
+    let input = /** @type{HTMLTextAreaElement} */
+        (document.getElementById('input'));
+    input.placeholder = 'Type /help for help';
+    setTimeout(() => {input.placeholder = '';}, 8000);
+
     this.request(getSettings().request);
 
     if(serverConnection.permissions.present && !findUpMedia('local')) {