Integrate Monolog: replace four logging systems with single PSR-3 factory

- Add monolog/monolog dependency (^3.10)  
- Create app/Logger.php central factory with channels: app, admin, error, audit
- Each channel gets RotatingFileHandler (30-day retention) with pass-through LineFormatter
  preserving existing JSON format contracts
- Rewrite AppLogger as thin facade delegating to Logger::get('app')
- Rewrite ErrorHandler::log() to delegate to Logger::get('error')
- Rewrite AdminLogger file output to delegate to Logger::get('admin'), keep DB writes
- Add Monolog file shadow to Audit via Logger::get('audit') (Option A per monolog-plan)
- Log level controlled by LOG_LEVEL env var (defaults: DEBUG in cli-server, WARNING otherwise)
- Graceful NullHandler fallback when log directory is not writable
- Update SystemController LOG_FILES: remove php_error, add app/admin/error/audit
- JSON app logs parsed to readable one-liners in the log viewer
- Remove nginx config tab (parametres + fragment + template + css)
- Friendly empty-state message when app log files don't exist yet (notYet)
- PHP tail fallback when exec() unavailable
- All 228 PHPUnit tests pass, no call sites changed

app/templates/admin/parametres.php

@@ -472,23 +472,10 @@
                 <?= htmlspecialchars($def['label']) ?>
             </a>
             <?php endforeach; ?>
-            <a href="?tab=nginx_config"
-               class="sys-tab <?= $activeTab === 'nginx_config' ? 'active' : '' ?>"
-               hx-get="/admin/system-fragment.php?tab=nginx_config"
-               hx-target="#sys-tab-panel"
-               hx-push-url="?tab=nginx_config"
-               hx-swap="innerHTML"
-               hx-indicator="#sys-tab-panel"
-               data-tab="nginx_config"
-               <?= $activeTab === 'nginx_config' ? 'aria-current="page"' : '' ?>>nginx — config</a>
         </nav>
 
         <div id="sys-tab-panel">
-            <?php if ($activeTab === 'nginx_config'): ?>
-                <?php include APP_ROOT . '/templates/admin/partials/system-nginx-config-panel.php'; ?>
-            <?php else: ?>
-                <?php include APP_ROOT . '/templates/admin/partials/system-log-panel.php'; ?>
-            <?php endif; ?>
+            <?php include APP_ROOT . '/templates/admin/partials/system-log-panel.php'; ?>
         </div>
     </section>
     </article>