Add markdown file caching functionality

Add cache.php with get/set functions for markdown content Update
rendering to check cache before processing files

app/cache.php

@@ -0,0 +1,27 @@
+<?php
+
+function getCachedMarkdown(string $filePath): ?string {
+    $cacheDir = '/tmp/folderweb_cache';
+    $mtime = filemtime($filePath);
+    $cacheKey = md5($filePath . $mtime);
+    $cachePath = "$cacheDir/$cacheKey";
+    
+    if (file_exists($cachePath)) {
+        return file_get_contents($cachePath);
+    }
+    
+    return null;
+}
+
+function setCachedMarkdown(string $filePath, string $html): void {
+    $cacheDir = '/tmp/folderweb_cache';
+    if (!is_dir($cacheDir)) {
+        mkdir($cacheDir, 0755, true);
+    }
+    
+    $mtime = filemtime($filePath);
+    $cacheKey = md5($filePath . $mtime);
+    $cachePath = "$cacheDir/$cacheKey";
+    
+    file_put_contents($cachePath, $html);
+}