Self-host front-end libraries, minimal sign-in header and CSP

- sign-in pages load only what they use (include_login_header.php, ajax_core.js)
- every library, font and data file served from assets/vendor with pinned
  versions (VERSIONS.json); no CDN at runtime
- Content-Security-Policy on app, sign-in and landing pages
- remove httpbin Dropzone target and source-map references

landing/privacy.php

@@ -1,5 +1,6 @@
 <?php
 require __DIR__ . '/../app/config.php';
+require __DIR__ . '/includes/security_headers.php';
 $base    = rtrim($base_url, '/') . '/landing';
 $app_url = rtrim($base_url, '/') . '/app';
 $page_title       = 'Privacy Policy — BRN WMS';