Clicking an invitation link while signed out dropped the visitor on a bare login form,
even though most invitees have no account yet and nothing on screen told them to create one.
Signed-out visitors now get the invitation itself: who invited them, which workspace/project,
which role, and which address it was sent to. The primary call to action follows whether an
account already exists for that address — "Create your account" when it does not, "Sign in to
accept" when it does.
The sign-up path carries the invitation forward, so a new account lands back on the invitation
and from there on the shared workspace/project instead of the onboarding wizard:
- the register link passes invitationToken, the invited email and a callbackUrl
- the register form locks the email to the invited address and shows what is being joined
- the verification email round-trips the destination through a sanitized `next` parameter
- login and verify-email keep the pending destination in their sign-in links
Signing in with a different address than the one invited now explains the mismatch instead of
silently redirecting to the dashboard.
Callback sanitization moves to lib/safe-redirect.ts so login, register, verify-email and the
verification route share one open-redirect guard.
yusufipk