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/login.html

@@ -70,7 +70,7 @@ document.addEventListener('DOMContentLoaded', () => {
       const user = await API.users.me();
       Auth.saveUser(user);
 
-      window.location.href = '/dashboard.html';
+      window.location.href = '/index.html';
     } catch (err) {
       alert.innerHTML = `<div class="alert alert-error">${err.message}</div>`;
       btn.disabled = false;