Fix: pressing Enter on the Password tab in the MCP authorization screen now signs in with the password instead of emailing an OTP

On the OAuth/MCP authorization screen, typing your email and password and pressing Enter would silently email you a 6-digit code and drop you on the code-entry screen instead of signing you in. Clicking "Sign in & approve" always worked; only the Enter key was broken. The fix disables the inactive submit buttons so implicit form submission fires the correct action for whichever tab and state is active. The "No Vello account found for that email" error copy is also corrected to "No memsprout account found for that email."