Show inline "email already exists" error on sign-up instead of activation email prompt

Current behavior

Signing up with an email that already has an account shows the "check your email to activate your account" message. The user gets no indication that the email is already registered.

Proposed change

Show an inline error immediately on submit:

• "An account with this email already exists. Sign in instead."
• A link to the login page.
• A "Forgot your password?" link pointing to the reset flow.

No email should be sent. No redirect. The error appears where the form is.

Impact

Affects Cloud users who try to create a second account or who forgot they already signed up. Currently they wait for an email that explains nothing and may not arrive if the address is already verified.