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/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>