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

app/ac_dashboard/index.php

@@ -179,7 +179,7 @@
 
   <?php require '../include_ending.php'; ?>
 
-  <script src="https://cdn.jsdelivr.net/npm/chart.js@4/dist/chart.umd.min.js"></script>
+  <script src="<?php echo $server_url?>assets/vendor/chart.js/4.5.1/chart.umd.min.js"></script>
   <style>
   /* Brief yellow flash when a card updates silently via WebSocket */
   @keyframes card-flash {