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/become-a-member/index.html

@@ -347,8 +347,9 @@
                 <p>A few friendly questions. You do not need to decide now how active you want to be.</p>
             </div>
             <form class="ms-form" novalidate>
+                <div class="ms-hp" aria-hidden="true"><label>Do not fill this in<input type="text" name="company" tabindex="-1" autocomplete="off"></label></div>
                 <div class="ms-progress" data-step="Step" data-of="of" hidden><div class="ms-progress-track"><div class="ms-progress-bar"></div></div><span class="ms-progress-text"></span></div>
-                <div class="ms-alert" hidden>Please fill in the required fields.</div>
+                <div class="ms-alert" hidden data-required-msg="Please fill in the required fields." data-cooldown-msg="It looks like you have just sent a request. Please give us a moment before sending another.">Please fill in the required fields.</div>
 
                 <div class="ms-step" data-step="path" data-flow="both">
                     <h3>How would you like to connect with MSOS?</h3>
@@ -567,10 +568,10 @@
             </div>
         </div>
     </div>
-    <script src="../../main.js"></script>
 </footer>
 <!-- END_FOOTER -->
     <script src="../../member-form.js"></script>
 
+    <script src="../../main.js"></script>
 </body>
 </html>