Turn admin lobby chat panel into a tab

The lobby chat was a standalone card sitting above the per-player log
tabs. Folds it into the same tab bar as the first (default-active)
tab instead, so the session log view has one consistent tab strip.
The tab-switching script now toggles by an .admin-tab-panel class
instead of assuming every panel's id starts with "player-", since
that's no longer true.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/admin-session-tabs.js

@@ -5,7 +5,7 @@ document.addEventListener('DOMContentLoaded', () => {
     }
 
     const activate = (id) => {
-        document.querySelectorAll('[id^="player-"]').forEach(el => el.style.display = 'none');
+        document.querySelectorAll('.admin-tab-panel').forEach(el => el.style.display = 'none');
         const target = document.getElementById(id);
         if (target) {
             target.style.display = 'block';