Fix duplicate and empty categories in nav filter

Trim whitespace after splitting on comma and filter out empty strings
before deduplication, so categories like " Imprimés" and "Imprimés"
are correctly unified.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

site/controllers/index.php

@@ -22,8 +22,10 @@ return function($page) {
             return explode(',', $field);
         }, $categoriesRawFields); // return ’[["A", "B", "C",], ["A", "B"]]
         $flattenedCategories = array_flatten($splittedCategoriesFields);
-        $filters = array_unique($flattenedCategories);    
-        
+        $trimmed = array_map('trim', $flattenedCategories);
+        $filtered = array_filter($trimmed, fn($v) => $v !== '');
+        $filters = array_unique($filtered);
+
         return $filters;
     }