Add El source files for all client-side JS

Recovers original JS from git history and ports it into proper El source
files under src/js/. Each file wraps the original JS in a native_js call
inside a main() function, making it valid El that compiles to a
self-contained IIFE via elc --target=js --bundle.

Files added:
  src/js/account-auth.el       - Supabase OTP magic-link (sendMagicLink)
  src/js/account-dashboard.el  - Account dashboard: session, plan card, family
  src/js/chat-widget.el        - Demo chat widget (neuronDemoToggle/Send/Reset)
  src/js/checkout-auth.el      - Checkout auth: OAuth, email sign-in/up
  src/js/checkout-free.el      - Free plan: auth-badge watch -> payment reveal
  src/js/checkout-stripe.el    - Stripe Payment Element (reads NEURON_CFG)
  src/js/enterprise.el         - Enterprise inquiry form + headcount filter
  src/js/environmental.el      - Efficiency calculator slider
  src/js/gallery.el            - Gallery nav, search/sort, Supabase voting
  src/js/main.el               - Share page voting + copyForPlatform
  src/js/marketplace.el        - Developer interest form
  src/js/nav.el                - Nav hamburger + Mission dropdown
  src/js/styles.el             - Landing: nav scroll, reveal, founding counter

src/js/checkout-free.el

@@ -0,0 +1,17 @@
+// checkout-free.el -- Free plan: reveal payment section after auth completes.
+// Watches the auth-badge element; when it becomes visible, shows payment-section.
+// Compiled with: elc --target=js --bundle --minify --obfuscate
+
+fn main() -> Void {
+    native_js("(function() {
+  var pay = document.getElementById('payment-section');
+  if (!pay) return;
+  var timer = setInterval(function() {
+    var badge = document.getElementById('auth-badge');
+    if (badge && badge.offsetParent !== null) {
+      pay.style.display = '';
+      clearInterval(timer);
+    }
+  }, 150);
+})()")
+}