Refactor API structure: add internal handling, update templates, and remove deprecated files

public/templates/standard/error/error.html

@@ -0,0 +1,20 @@
+<!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 class="text">
+            <h2>Error</h2>
+            <p>%error%</p>
+        </div>
+    </div>
+</body>
+</html>