Add about page, update login redirect, and hero button tweaks

- Add about.html explaining infinity bottles, why to track, how to start
- Hero shows "What is it?" + "Track Your Bottle" when logged out; hides about button when logged in
- Redirect after login goes to index (community page) instead of dashboard
- redirectIfLoggedIn also sends to index

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/index.html

@@ -21,7 +21,10 @@
   <div class="hero">
     <h1>The Infinity Bottle</h1>
     <p>One pour from every bottle. An ever-evolving blend that grows richer with every addition.</p>
-    <a href="/login.html" class="btn btn-primary" id="hero-cta">Track Your Bottle</a>
+    <div style="display:flex;gap:1rem;justify-content:center;flex-wrap:wrap">
+      <a href="/about.html" class="btn btn-ghost" id="hero-about">What is it?</a>
+      <a href="/register.html" class="btn btn-primary" id="hero-cta">Track Your Bottle</a>
+    </div>
   </div>
 
   <div class="section-header">
@@ -56,6 +59,7 @@ document.addEventListener('DOMContentLoaded', async () => {
   if (Auth.isLoggedIn()) {
     document.getElementById('hero-cta').textContent = 'Go to My Bottle';
     document.getElementById('hero-cta').href = '/dashboard.html';
+    document.getElementById('hero-about').style.display = 'none';
   }
 
   const container = document.getElementById('user-cards');