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]>

docs/features/editor-blocks.md

@@ -29,7 +29,8 @@ Four blocks have sidebar (inspector) options:
 | `us-scheduler/booking` | `autoRedirect` (boolean) | `false` | Send logged-out visitors straight to the login page instead of showing the link. |
 | `us-scheduler/student-login` | `bookingPageId` (number) | `0` | Page the "View available lessons" link points to for logged-in visitors, and the post-login redirect target. `0` = the current page. |
 | `us-scheduler/student-login` | `autoRedirect` (boolean) | `false` | Send logged-in visitors straight to the booking page instead of showing the link. Does nothing until a booking page is chosen. |
-| `us-scheduler/student-register` | `loginPageId` (number) | `0` | Page the "Sign in to your account" link points to after a student confirms their email. `0` = the WordPress login screen. Shortcode equivalent: `[us_student_register login_page_id="…"]`. |
+| `us-scheduler/student-register` | `loginPageId` (number) | `0` | Page students continue to once registration finishes — the "Sign in to your account" link after they confirm their email, and the "Continue to your account" link an invited student gets on the spot. `0` = the WordPress login screen for the confirmation link, and no link at all for the (already signed-in) invited student. Shortcode equivalent: `[us_student_register login_page_id="…"]`. |
+| `us-scheduler/student-register` | `autoRedirect` (boolean) | `false` | Send students straight to that page instead of showing the link. Does nothing until a page is chosen — there is no login-screen fallback here. |
 | `us-scheduler/group-classes` | `offeringId` (number) | `0` | Restrict the page to a single group class, for embedding on a page dedicated to that class. The class description is then omitted — only the schedule, instructor, price and enrolment controls are shown, so the surrounding page's own copy is not repeated. `0` = browse all classes, descriptions included. Shortcode equivalent: `[us_group_classes offering="…"]`. |
 
 The page selects list all published pages; if a chosen page is later deleted,
@@ -48,6 +49,15 @@ queried singular post's content for the block (including inside nested
 blocks), and redirects when the block opts in. A block whose target is its
 own page is ignored to avoid a redirect loop.
 
+The registration block's auto-redirect additionally only fires on a
+**finished** registration — `RegistrationPage::isRegistrationComplete()`: an
+invited student who is now logged in (`?us_registered=invite`), or a
+self-signup back from the emailed confirmation link (`?us_confirmed=ready|1`).
+The intermediate "check your email" step and every failure (a validation
+error, `?us_confirmed=expired`) stay on the page so the student reads the
+message. That check runs before the content is parsed, so an ordinary page
+view does not pay for the extra block scan.
+
 ## How it works
 
 - **`BlockRegistrar`** (`src/BlockRegistrar.php`) hooks `init` and registers