fix(frontend): restore chat DOM structure and guard addMessage against null

frontend/index.html

@@ -41,7 +41,7 @@
           <div class="chat-header-actions">
             <span id="chat-user-label" class="chat-user-label" hidden></span>
             <button id="auth-toggle-btn" type="button" class="btn btn-secondary">Sign in</button>
-            <button id="auth-logout-btn" type="button" class="btn btn-secondary" hidden>Sign out</button>              <button id="users-toggle-btn" type="button" class="btn btn-secondary" aria-controls="presence-panel" aria-expanded="true">Users</button>            <button type="button" class="btn btn-secondary" onclick="togglePrivateDM()">New DM</button>
+            <button id="auth-logout-btn" type="button" class="btn btn-secondary" hidden>Sign out</button> <button id="users-toggle-btn" type="button" class="btn btn-secondary" aria-controls="presence-panel" aria-expanded="true">Users</button> <button type="button" class="btn btn-secondary" onclick="togglePrivateDM()">New DM</button>
           </div>
         </header>
 
@@ -55,11 +55,14 @@
           <p id="auth-status" class="status-line" aria-live="polite"></p>
         </div>
 
-          <div id="dm-search-row" class="dm-search-row" hidden>
-            <input id="dm-search-input" type="text" class="input" placeholder="Search this conversation..." />
-            <button id="dm-search-btn" type="button" class="btn btn-secondary">Search</button>
-          </div>
+        <section id="messages" class="chat-messages" aria-live="polite"></section>
 
+        <div id="dm-search-row" class="dm-search-row" hidden>
+          <input id="dm-search-input" type="text" class="input" placeholder="Search this conversation..." />
+          <button id="dm-search-btn" type="button" class="btn btn-secondary">Search</button>
+        </div>
+
+        <section id="presence-panel" class="presence-panel">
           <div id="presence-list" class="presence-list"></div>
         </section>