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();

public/index.html

@@ -31,6 +31,12 @@
                         <label for="password">Password:</label>
                         <input type="password" id="password" name="password" required>
                     </div>
+                    <div class="form-group form-group--checkbox">
+                        <label class="checkbox-label">
+                            <input type="checkbox" id="remember-me" name="rememberMe">
+                            <span>Keep me logged in</span>
+                        </label>
+                    </div>
                     <button type="submit" class="login-btn">Login</button>
                 </form>
                 <div id="login-error" class="error-message" style="display: none;"></div>

public/style.css

@@ -612,6 +612,32 @@ body {
   border-color: var(--accent);
 }
 
+.form-group--checkbox {
+  margin-bottom: 20px;
+}
+
+.checkbox-label {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  cursor: pointer;
+  font-size: 0.9rem;
+  color: var(--text-secondary);
+  user-select: none;
+}
+
+.checkbox-label input[type="checkbox"] {
+  width: 16px;
+  height: 16px;
+  accent-color: var(--accent);
+  cursor: pointer;
+  flex-shrink: 0;
+}
+
+.checkbox-label span {
+  line-height: 1;
+}
+
 .login-btn {
   width: 100%;
   padding: 10px;