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

composer.json

@@ -6,10 +6,11 @@
     "require": {
         "php": ">=8.4",
         "ext-json": "*",
-        "ext-pdo": "*",
         "ext-openssl": "*",
-        "league/commonmark": "^2.4",
+        "ext-pdo": "*",
         "guzzlehttp/guzzle": "^7.9",
+        "league/commonmark": "^2.4",
+        "monolog/monolog": "^3.10",
         "phpmailer/phpmailer": "^6.9"
     },
     "require-dev": {