feat(ui): shared header — Sign in/Sign out and New Chat on every page; hide the Sources nav link from anonymous users

frontend/login.html

@@ -20,7 +20,11 @@
       </span>
       <nav class="app-nav" aria-label="Primary">
         <a href="/" class="nav-link">Chat</a>
-        <a href="/sources.html" class="nav-link">Sources</a>
+        <!-- Phase 19: the Sources link is admin-only (owner permission
+             2026-08-23) — hidden by default, header.js reveals it once
+             whoami says admin. The login page deliberately carries NO
+             chat controls, so header.js only toggles this link here. -->
+        <a href="/sources.html" class="nav-link" id="nav-sources" hidden>Sources</a>
       </nav>
     </div>
   </header>
@@ -57,6 +61,10 @@
     </div>
   </footer>
 
+  <!-- Phase 19: shared header module — the login page reuses it for the
+       Sources-link toggle only (no chat controls in this markup, so
+       none appear). -->
+  <script type="module" src="/assets/header.js"></script>
   <script type="module" src="/assets/login.js"></script>
 </body>
 </html>