Add Dry Run button; fix CSRF token on all POST API calls

All fetch() POSTs now send application/x-www-form-urlencoded with the
page-injected csrf_token, satisfying unRAID's auto_prepend CSRF check.
All PHP API handlers switched from php://input JSON to $_POST.

Also adds Dry Run button (orange, between Run and Log) that sets
DRY_RUN=1 in the script environment before executing.

Plugin/usr/local/emhttp/plugins/varaverk/api/settings.php

@@ -1,8 +1,7 @@
 <?php
 header('Content-Type: application/json');
 
-$body      = json_decode(file_get_contents('php://input'), true);
-$scriptsDir = trim($body['scripts_dir'] ?? '');
+$scriptsDir = trim($_POST['scripts_dir'] ?? '');
 
 if (!$scriptsDir) {
     echo json_encode(['ok' => false, 'error' => 'scripts_dir is required']);