feat: add 'Keep me logged in' checkbox to login form

- index.html: checkbox between password field and login button
- app.js: reads #remember-me and passes rememberMe in POST body
- auth.js: rememberMe=true sets 30-day maxAge; false = session cookie
  (expires when browser closes)
- style.css: .form-group--checkbox and .checkbox-label styles

public/app.js

@@ -136,6 +136,7 @@ async function handleLogin(e) {
   
   const username = document.getElementById('username').value;
   const password = document.getElementById('password').value;
+  const rememberMe = document.getElementById('remember-me').checked;
   
   try {
     const response = await fetch('/api/auth/login', {
@@ -143,7 +144,7 @@ async function handleLogin(e) {
       headers: {
         'Content-Type': 'application/json'
       },
-      body: JSON.stringify({ username, password })
+      body: JSON.stringify({ username, password, rememberMe })
     });
     
     const data = await response.json();