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/partials/system-log-panel.php

@@ -19,7 +19,7 @@
 
 <?php if ($logFileMeta): ?>
 <div class="log-meta">
-    <span data-label="Fichier"><?= htmlspecialchars(SystemController::LOG_FILES[$activeTab]['path']) ?></span>
+    <span data-label="Fichier"><?= htmlspecialchars($logFileMeta['path']) ?></span>
     <span data-label="Taille"><?= $logFileMeta['size'] ?></span>
     <span data-label="Modifié"><?= $logFileMeta['mtime'] ?></span>
 </div>
@@ -29,7 +29,7 @@
     <div class="log-unavailable">
         <strong>Journaux non disponibles</strong>
         <div class="log-unavail-path"><?= $logError ?></div>
-        <?php if (php_sapi_name() === 'cli-server'): ?>
+        <?php if (php_sapi_name() === 'cli-server' && str_starts_with($activeTab, 'nginx')): ?>
         <div class="log-unavail-dev">
             En environnement de développement, les logs nginx ne sont pas disponibles.
             Cette page est pleinement fonctionnelle sur le serveur de production.
@@ -37,6 +37,12 @@
         <?php endif; ?>
     </div>
 
+<?php elseif (!empty($notYet)): ?>
+    <div class="log-empty">
+        Aucune entrée pour le moment.<br>
+        <span class="log-unavail-path">Le journal sera créé automatiquement au premier événement.</span>
+    </div>
+
 <?php elseif (empty($logLines)): ?>
     <div class="log-empty">Le fichier journal est vide.</div>