Let the studio register the students who cannot register themselves

The Book a lesson for a student panel built its picker from the us_student
role but vetted the submission with the book_lesson capability. ChildLoginGate
and RegistrationLoginGate withhold that capability from accounts that keep the
role, so the panel offered every guardian-managed child and every unapproved
signup and then refused them — with a message claiming no student had been
chosen, and a form cleared of all five fields.

Withholding book_lesson stops those accounts registering in their own name. It
was never meant to stop the studio acting for them, which is what the panel is
for, and for a child is the only route to a lesson besides their guardian.

Guard the student role instead, via a new RoleManager::isStudent() shared with
every picker and guard on the staff side so the two cannot drift apart again.
Group enrolment gets the same predicate: addDirect() and grantAccess() vetted
their posted ids not at all, and would enrol an instructor, an administrator,
or an account deleted since the page was drawn — raising a real payment against
them for a priced class.

Keep a refused booking's fields as submitted, reading the form through one
LessonController::submittedBooking() so what gets booked and what is shown
again cannot disagree about a field name. A booking that succeeds still leaves
an empty form, so the next one does not inherit it.

Closes #185

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01XunBYk2sFEc1oL14sUiuBU

CHANGELOG.md

@@ -13,6 +13,11 @@ each change under the current top section as you work.
 
 ## [1.5.4]
 
+### Fixed
+- **Adding students to a group class now checks that each one is actually a student.** **Add students directly** and **Make available** acted on whatever ids the form posted without confirming they named students at all, so a stale page — or a tampered submission — could put an instructor, an administrator, or an account that had since been deleted onto a class roster, raising a real payment against them. Both controls now skip anything that is not a student, and the "%d student(s) added" count tells you how many actually went through. Children and students awaiting approval are unaffected: they are students, and adding them is what these controls are for.
+- **A refused booking no longer empties the Book a lesson for a student form.** Whatever the reason it came back — the time taken while you were typing, a weekly reservation asked for on a time that does not repeat — the panel reopens with the student, time, lesson type, both ticks and your note exactly as you left them, so a correction is one field, not five. A booking that goes through still leaves an empty form behind for the next one.
+- **Booking a lesson for a child, or for a student you have not approved yet, no longer fails with "Choose a student to book for."** The **Book a lesson for a student** panel offered every student it could see, but then refused a good half of them: a parent's child and a self-signup still awaiting approval both appeared in the list, and both were rejected on submit — with an error that read as though no student had been chosen, and which cleared the form. Neither of those accounts is allowed to book *in their own name* (a child's is never signed in to at all, and an unapproved signup waits for you), and the panel was mistakenly applying that same restriction to the studio booking on their behalf, which is precisely the case it was built for. Anyone the panel offers can now be booked for.
+
 ## [1.5.3]
 
 ### Added