Let a guardian enrol every child in the same group class

The group-class page matched enrolments to the account instead of to the
student: the lookup it built from GET /enrollments was keyed by offering id
alone, so the first household enrolment marked the class as "yours" and took
the Enrol button away from everyone else on the account. A parent could enrol
one child and was then offered nothing but Withdraw.

The server was never the constraint — hasActiveEnrollment() checks the
(offering, student) pair and GET /enrollments deliberately returns the whole
household — so the fix is to stop discarding student_id on the way in. Active
enrolments are now grouped per class as a list, each student gets their own
"… is enrolled in this class." line and their own named Withdraw button, and
the Enrol button stays (as "Enrol another student") while anyone the account
may enrol is still out.

The enrolment form offers only the students not yet enrolled. When exactly one
of them is left the picker collapses, and that case needed care: an omitted
student_id reads as "enrol the account holder" server-side, so a hidden field
carries the id rather than posting nothing and signing up the parent instead of
the last child.

Closes #170

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Jy9UPhmpLUAfimsyecHN2Z

CHANGELOG.md

@@ -13,6 +13,9 @@ each change under the current top section as you work.
 
 ## [1.5.1]
 
+### Fixed
+- **A parent can now enrol more than one child in the same group class.** Enrolling the first student worked, and then the class card switched to "You are enrolled in this class." with a **Withdraw** button — for the whole account. There was no way to sign up a second child short of withdrawing the first, even though nothing was ever actually full or forbidden: the class page was matching enrolments to the account rather than to the student, so one child's seat spoke for everybody. Each enrolled student now gets their own line on the card, named — "Ada is enrolled in this class." — with their own Withdraw button, and the Enrol button stays put, reading **Enrol another student**, until everyone on the account is in. The form's "Who is this for?" list offers only the students not yet enrolled, so the class cannot be double-booked for the same child by accident. Enrolments already recorded are unaffected; the seats were always separate on the studio's side, and this is the page catching up with that.
+
 ## [1.5.0]
 
 ### Added