Refactor template and content handling logic

Add constants for file extensions

Extract helper functions for common operations

Improve PDF file detection

Simplify directory scanning operations

Standardize template resolution

Optimize content rendering pipeline

app/constants.php

@@ -0,0 +1,7 @@
+<?php
+
+// Content file extensions
+define('CONTENT_EXTENSIONS', ['md', 'html', 'php']);
+
+// Cover image extensions
+define('COVER_IMAGE_EXTENSIONS', ['jpg', 'jpeg', 'png', 'webp', 'gif']);