feat(admin): add transparent debug error-log mode

Add an admin-controlled debug mode that captures PHP errors of all
players into var/log/debug-players.log, to hunt remaining PHP 8.3 bugs
from real play sessions. Fully transparent to players: no redirect, no
gameplay change, errors are never displayed.

- DB: new debug_log table (one row), mirroring the maintenance pattern.
- Database: getDebugMode()/setDebugMode()/setDebugSettings(), defensive
  when the table is absent (no blank page).
- Session: register a custom error + shutdown handler when enabled; the
  handler runs even when php.ini error_reporting masks warnings/notices,
  so capture is complete without a Docker rebuild. Auto-disables after a
  configurable window.
- DebugErrorLogger: size-capped file with a single .log.1 rotation,
  honours the @ operator, never throws.
- Admin: new "Debug Error Log" page (levels, size cap, auto-off, on-page
  viewer, clear, download) + debugLog action mod.
- Menu: admin-only quick on/off widget (TZ_DEBUG_ON/OFF, EN/FR/RO).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Admin/admin.php

@@ -147,6 +147,10 @@ if (!empty($_GET['p'])) {
             $subpage = 'Server Settings';
             break;
 
+        case 'debug_log':
+            $subpage = 'Debug Error Log';
+            break;
+
         case 'editServerSet':
             $subpage = 'Server Configuration';
             break;
@@ -622,6 +626,7 @@ body.app #lmid3 a{color:#15803d !important;font-weight:600 !important;}
 						<li class="sub"><a href="#">Admin</a>
 							<ul>
 								<li><a href="?p=admin_log"><font color="Red"><b>Admin Log</b></font></a></li>
+								<li><a href="?p=debug_log">Debug Error Log</a></li>
 								<li><a href="?p=config">Server Settings</a></li>
 								<li><a href="?p=maintenance">Server Maintenance</a></li>
 								<li><a href="?p=resetServer">Server Resetting</a></li>