Simple HTML Login Mockup

I need a lightweight, front-end-only user login demo built with pure HTML, CSS and vanilla JavaScript. The flow is straightforward: a visitor sees a login screen, enters a username and password, the script checks that both fields are at least six characters long, and—if the check passes—redirects the user to a protected “welcome” page that includes a visible Logout button. Hitting Logout should clear whatever flag you set in sessionStorage or localStorage and return the user to the original login form.

Because this is just a mock-up, no server-side code or databases are involved; all logic stays in the browser. Keep the markup semantic, the CSS clean and responsive, and comment the JavaScript so I can follow the validation and simple session handling.

Please package the final solution in a single zipped folder containing the HTML, a separate CSS file, and a JS file, ready for me to drop into a browser and test immediately.