Add meta description support to pages

Add description meta tag to base template Fix navigation list structure
in header Add meta description extraction helper function Update
rendering and routing to include meta description

app/helpers.php

@@ -64,3 +64,43 @@ function findPdfFile(string $dirPath): ?string {
     $pdfs = glob("$dirPath/*.pdf") ?: [];
     return $pdfs ? basename($pdfs[0]) : null;
 }
+
+function extractMetaDescription(string $dirPath, ?array $metadata, string $lang, string $defaultLang): ?string {
+    // 1. Check for search_description in metadata
+    if ($metadata && isset($metadata['search_description'])) {
+        return $metadata['search_description'];
+    }
+    
+    // 2. Fall back to summary in metadata
+    if ($metadata && isset($metadata['summary'])) {
+        return $metadata['summary'];
+    }
+    
+    // 3. Fall back to first paragraph in content files
+    $files = findAllContentFiles($dirPath, $lang, $defaultLang, []);
+    if (empty($files)) return null;
+    
+    foreach ($files as $file) {
+        $ext = pathinfo($file, PATHINFO_EXTENSION);
+        $content = file_get_contents($file);
+        
+        if ($ext === 'md') {
+            // Skip headings and extract first paragraph
+            $lines = explode("\n", $content);
+            foreach ($lines as $line) {
+                $line = trim($line);
+                if (empty($line) || str_starts_with($line, '#')) continue;
+                if (strlen($line) > 20) { // Ignore very short lines
+                    return strip_tags($line);
+                }
+            }
+        } elseif (in_array($ext, ['html', 'php'])) {
+            // Extract first <p> tag content
+            if (preg_match('/<p[^>]*>(.*?)<\/p>/is', $content, $matches)) {
+                return strip_tags($matches[1]);
+            }
+        }
+    }
+    
+    return null;
+}