feat: add account selector dropdown in message viewer

webui/viewer.html

@@ -13,7 +13,7 @@
           <div>
             <div class="eyebrow">Export Viewer</div>
             <h1>Messages</h1>
-            <p class="muted small">Account: <span id="viewer-account-name">Loading...</span></p>
+            <p class="muted small">Account: <select id="viewer-account-select"></select></p>
           </div>
           <button id="sidebar-toggle" class="button button-small sidebar-toggle">☰</button>
           <a class="button button-small" href="/">Dashboard</a>