Signup form with inline email validation

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Build a self-contained signup form as a single HTML file (`index.html`) that renders with no build step and no network calls (inline all CSS and JS). Requirements: - an email input and a submit button inside a form, - inline client-side validation that runs on submit and does not reload the page: submitting with an empty or invalid email reveals a visible error message (use role="alert" for the error), and the input is marked aria-invalid="true", - submitting a valid email reveals a visible success message (use role="status") and clears the error. Do not rely on the browser's native required-field popup; show your own inline message element. Use plain, readable, accessible markup. No external fonts or scripts.