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/assets/css/main.css

@@ -1,4 +1,4 @@
-@charset "UTF-8";@import"https://fonts.googleapis.com/css2?family=Poppins:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;0,800;0,900;1,100;1,200;1,300;1,400;1,500;1,600;1,700;1,800;1,900&display=swap";@import"https://cdnjs.cloudflare.com/ajax/libs/tabler-icons/3.35.0/tabler-icons.min.css";/*!
+@charset "UTF-8";@import"../vendor/fonts/poppins/poppins.css";@import"../vendor/tabler-icons/3.35.0/tabler-icons.min.css";/*!
  * Bootstrap  v5.3.8 (https://getbootstrap.com/)
  * Copyright 2011-2025 The Bootstrap Authors
  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/main/LICENSE)