Enhance HTMLLoader to include CSS files in error templates and add error handling for missing CSS. Define APP_DOMAIN in main.php and update error template structure.

public/templates/error/error.css

@@ -0,0 +1,3 @@
+.icon {
+    font-size: 100pt;
+}

public/templates/error/error.html

@@ -1 +1,16 @@
-<h1>%error%</h1>
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Error</title>
+    %css%
+</head>
+<body>
+    <div class="page-content">
+        <div class="icon">
+            ⚠️
+        </div>
+    </div>
+</body>
+</html>