Send students to a chosen page when registration succeeds

The Student Registration block's "After email confirmation" panel becomes
"After registration": the page it selects is now where a newly registered
student continues to, and a new autoRedirect toggle sends them there
instead of showing the link.

Only the two finished states qualify (RegistrationPage::isRegistrationComplete):
an invited student who is now logged in, and a self-signup back from the
emailed confirmation link. A validation error, an expired confirmation
link, and the intermediate "check your email" step all stay on the page so
their message is read.

The invited-student success previously had no link at all; it gains a
"Continue to your account" one. That path deliberately has no
WordPress-login-screen fallback — pointing someone already signed in at the
login screen helps nobody — so continueUrl() distinguishes "no page chosen"
from "page chosen", and the redirect does nothing until one is picked.

Closes #115

Co-Authored-By: Claude Opus 5 <[email protected]>

CHANGELOG.md

@@ -15,6 +15,7 @@ each change under the current top section as you work.
 
 ### Added
 - The **Group Classes** block can now be pinned to a single class, under **Classes shown → Class** in the block sidebar (shortcode: `[us_group_classes offering="…"]`). Pick a class and the block shows only that one, so it can be embedded on a page that describes the class. In this mode the class's own description is left out to avoid repeating the page copy — the card shows the schedule, instructor, price, enrolment deadline and the enrol/withdraw controls. Leaving it on **All classes** keeps the full browsable catalog with descriptions.
+- The **Student Registration** block can now send students onward to a page of your choosing once they finish registering. Its **After email confirmation** panel is now **After registration**: the page you pick there is where the link shown to a newly registered student points — the "Sign in to your account" link after they confirm their email, and a "Continue to your account" link for an invited student, who is signed in immediately. A new **Redirect automatically** option takes them straight there instead of showing the link. Registration errors are never skipped — a failed sign-up and an expired confirmation link still show their message on the page, as does the "check your email to confirm your address" step. The redirect needs a page to be chosen; with none set, students see the link (or, for invited students, just the confirmation) as before.
 
 ## [1.2.1]