Add Send button and inline keydown handler for chat

Belt-and-suspenders: inline onkeydown on textarea ensures Enter
works regardless of JS event listener registration order, plus
a visible Send button as clickable fallback.

static/css/style.css

@@ -425,6 +425,25 @@ p {
 /* Message input area */
 #message-form {
     margin-top: var(--space-3);
+    display: flex;
+    gap: var(--space-2);
+    align-items: flex-end;
+}
+
+#send-button {
+    padding: var(--space-2) var(--space-4);
+    background-color: var(--button-primary);
+    color: white;
+    border: none;
+    border-radius: var(--radius-md);
+    cursor: pointer;
+    font-size: 14px;
+    min-height: 60px;
+    white-space: nowrap;
+}
+
+#send-button:hover {
+    opacity: 0.9;
 }
 
 #message, .message-edit {

templates/chat.html

@@ -17,7 +17,8 @@
 
         <!-- Form for sending messages -->
         <form id="message-form">
-            <textarea id="message" placeholder="Type your message..."></textarea>
+            <textarea id="message" placeholder="Type your message..." onkeydown="if(event.key==='Enter'&&!event.shiftKey){event.preventDefault();sendMessage();}"></textarea>
+            <button type="submit" id="send-button">Send</button>
         </form>
     </div>