Let parents register once and book for their children

A parent registers once and manages lessons for one or more children, who
need no login of their own. A child is a real wp_users row with the student
role but no usable login — so student_id keeps meaning "a WordPress user"
on every table, and booking, credits, policies and enrolments work unchanged.
A us_guardians link table maps guardian to child.

The signup form gains a parent/guardian tick that reveals a block per child,
with the account-signup questions asked per child rather than per guardian
— they describe the student, not the account holder. Signup policies are
recorded once per child with the guardian as the acceptor, which is the
record that actually means something. A family that half-creates is rolled
back entirely rather than leaving a guardian who cannot re-register.

The booking and enrolment forms gain a "Who is this for?" picker listing
children first, so the default selection is never the parent — booking for
the wrong child is correctable, quietly billing a parent for their kid's
lesson is not. POST /bookings and POST /enrollments take an optional
student_id honoured only for that child's guardian; anything else is a 403.
That check is the authorisation boundary of the feature.

Payments and credits gain a payer: the charge names the child it was for and
the guardian who owes it, so per-child reporting is unchanged while notices,
receipts and the payment step reach the parent. Credit is held by the payer,
so one child's cancellation can settle a sibling's charge, and the daily
billing scan sends a guardian one notice covering every child.

Closes #132

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

assets/js/blocks.js

@@ -282,6 +282,28 @@
                 })
             ),
         },
+        {
+            name: 'us-scheduler/family',
+            title: __('Family', 'unsupervised-schedular'),
+            description: __('Lets a parent or guardian add, edit and remove the children they book lessons for.', 'unsupervised-schedular'),
+            icon: 'groups',
+            keywords: ['family', 'children', 'guardian', 'parent'],
+            shortcode: 'us_family',
+            attributes: {
+                loginPageId: { type: 'number', default: 0 },
+            },
+            inspector: (attributes, setAttributes) => el(
+                PanelBody,
+                { title: __('Logged-out visitors', 'unsupervised-schedular') },
+                el(PageSelect, {
+                    label: __('Login page', 'unsupervised-schedular'),
+                    help: __('Where visitors who are not signed in are sent to log in.', 'unsupervised-schedular'),
+                    defaultLabel: __('WordPress login screen', 'unsupervised-schedular'),
+                    value: attributes.loginPageId,
+                    onChange: (loginPageId) => setAttributes({ loginPageId }),
+                })
+            ),
+        },
     ];
 
     blocks.forEach((def) => {