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/js/auth.js

@@ -37,7 +37,7 @@ const Auth = (() => {
   }
 
   function redirectIfLoggedIn() {
-    if (isLoggedIn()) window.location.href = '/dashboard.html';
+    if (isLoggedIn()) window.location.href = '/index.html';
   }
 
   async function renderNav(activePage) {