introduce .auth-shell and .auth-card classes for the centered,
narrow-card layout shared by login and register, then apply them
to the login template. form ids and the #login-error element are
preserved so the existing cypress auth flows still target them.
add a stylesheet link and the standard responsive viewport meta tag
to every page template. purely additive - no markup or selector
changes - so existing cypress assertions and page js id hooks remain
intact. with this commit every page picks up the base styling from
app.css.