Fix mobile nav/TOC/overflow on article pages + harden membership form

Mobile fixes (article, guide, get-to-know, projects pages):
- Remove duplicate main.js include site-wide. footer.js emitted a
  <script src=main.js> inside the footer block while pages also load one
  before </body>, so DOMContentLoaded ran twice and every tap toggled the
  mobile menu / article TOC twice (net no-op). Menu and TOC only worked on
  the homepage. Now exactly one main.js per page, before </body>.
- Kill horizontal overflow: add overflow-x: clip to html/body (clips the
  off-canvas mobile nav panel without breaking position: sticky, unlike
  overflow-x: hidden).
- Reset align-items to stretch in the mobile .article-layout-container
  rule; the desktop flex-start made .article-container size to max-content
  width in a column flexbox and overflow the viewport.

Membership form hardening (Become a Member):
- Honeypot field + localStorage cooldown (30s gap, max 3/24h) + stronger
  email regex; trilingual cooldown alert message.
- Apps Script (setup doc): per-email auto-reply dedupe + daily send cap;
  applicant reply is a plain thank-you that never echoes submitted answers.
- Draft HTML welcome email (email-welcome-preview.html) for later use.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

en/student-welcome-guide/choose-a-programme/index.html

@@ -342,7 +342,6 @@
             </div>
         </div>
     </div>
-    <script src="../../../main.js"></script>
 </footer>
 <!-- END_FOOTER -->