a11y: make mobile menu toggle keyboard & screen-reader accessible

The mobile hamburger is a <div class="mobile-menu-icon"> with a click
handler, so it worked on tap but was invisible to keyboard and screen-reader
users. Expose it as a button in main.js: role="button", tabindex="0",
aria-label, aria-expanded toggling, and Enter/Space activation. Verified in
a headless browser (focusable, Enter opens, aria state flips).

Also adds docs/PRE-LAUNCH-CHECKLIST.md (dev-only, excluded from deploy):
inventory of remaining content placeholders for the public launch.

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

main.js

@@ -14,21 +14,42 @@ document.addEventListener('DOMContentLoaded', function() {
         if (!icon) {
             console.error('Mobile menu icon not found.');
         } else {
+            // Accessibility: the icon is a <div>, so expose it to keyboard and
+            // screen-reader users as a button (focusable, labelled, toggle state).
+            if (!icon.hasAttribute('role')) icon.setAttribute('role', 'button');
+            if (!icon.hasAttribute('tabindex')) icon.setAttribute('tabindex', '0');
+            if (!icon.hasAttribute('aria-label')) icon.setAttribute('aria-label', 'Open menu');
+            icon.setAttribute('aria-expanded', 'false');
+
+            const setMenu = (open) => {
+                root.classList.toggle('nav-open', open);
+                icon.setAttribute('aria-expanded', open ? 'true' : 'false');
+                icon.setAttribute('aria-label', open ? 'Close menu' : 'Open menu');
+            };
+
             icon.addEventListener('click', (e) => {
                 e.preventDefault();
                 e.stopPropagation();
-                root.classList.toggle('nav-open');
+                setMenu(!root.classList.contains('nav-open'));
+            });
+
+            // Activate with Enter / Space like a native button.
+            icon.addEventListener('keydown', (e) => {
+                if (e.key === 'Enter' || e.key === ' ' || e.key === 'Spacebar') {
+                    e.preventDefault();
+                    setMenu(!root.classList.contains('nav-open'));
+                }
             });
 
             document.addEventListener('click', (e) => {
                 if (root.classList.contains('nav-open') && !e.target.closest('.mobile-nav-panel') && !e.target.closest('.mobile-menu-icon')) {
-                    root.classList.remove('nav-open');
+                    setMenu(false);
                 }
             });
 
             document.addEventListener('keydown', (e) => {
                 if (e.key === 'Escape') {
-                    root.classList.remove('nav-open');
+                    setMenu(false);
                 }
             });
         }