Ask who the signup is for as a three-way choice

Replaces the single "I'm registering as a parent or guardian" tick with
"Just myself" / "On behalf of one or more students" / "Both".

Radios, not checkboxes as the feedback put it: the three answers are
mutually exclusive, and "both" only means anything as a third choice
alongside the other two.

The tick could only ever say whether there were children to add. It could
not say whether the account holder was a student, so bookableStudents()
always offered them their own name and any guardian could book themselves a
lesson nobody meant to sell. "On behalf of" now records us_guardian_only and
leaves them out of the picker.

That flag is stored as the negative on purpose. Every account predating this
choice is a bookable student, and absence has to keep meaning exactly that,
or the picker would quietly stop offering people themselves on upgrade.
setGuardianOnly() clears the key rather than writing 0, so "not set" stays
the single spelling of "yes, a student". A guardian-only account with nobody
linked to it is still offered itself — an empty picker is no way to book at
all, and they can put the account right from the profile page.

An unrecognised or absent value reads as "just myself": the choice that
collects the least and grants the least. A missing radio must never be taken
as "register these children".

Bumps to 1.4.0.

The account holder's own questions stay out of play whenever students are
being added, "both" included — asking them there is #146.

Verified the form in a headless browser across all three choices: which
blocks show, which fields carry `required`, whether the account holder's
question panel is disabled, which submit is offered, and that switching back
to "just myself" leaves no hidden required field blocking submit.

Closes #145

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

CHANGELOG.md

@@ -11,7 +11,7 @@ When a `v*` tag is pushed, `.gitea/workflows/release.yml` publishes the matching
 the plugin to the next patch version and adds a fresh section here for it. Record
 each change under the current top section as you work.
 
-## [1.3.1]
+## [1.4.0]
 
 ### Added
 - An **Account** block (`[us_account]`) showing who is signed in — their name and their email — and a **Sign out** link. Signing out returns to the login page chosen in the block, or to the page the visitor was already on when none is set, so putting it in a site header does not also move people somewhere. To a signed-out visitor it shows a **Sign in** link when a login page is chosen, and nothing at all when one is not: a panel about who is signed in has nothing to tell a stranger, and a notice they cannot act on is just clutter in a header.
@@ -20,6 +20,7 @@ each change under the current top section as you work.
 - Signup now checks the password properly. The form scores it as you type with the same zxcvbn meter wp-admin uses and will not submit a weak one, and the server refuses — regardless of what the browser allowed — anything shorter than 8 characters, one of the well-known leaked passwords, one built from barely any distinct characters, or one containing your own name or email address. Composition rules ("must contain a symbol") are deliberately not imposed: they mostly produce predictable substitutions. Email addresses are validated on the server on every signup path, with a clear message when one is already registered.
 
 ### Changed
+- Signup now asks **"Who are you registering?"** as a three-way choice — **just myself**, **on behalf of one or more students**, or **both** — in place of the single parent/guardian tick. The tick could only ever say "I have children to add"; it could not say whether the account holder was a student themselves, so every account was offered its own name in the **Who is this for?** picker whether or not anyone meant to book them a lesson. Choosing *on behalf of* now leaves the account holder out of that picker. Existing accounts are unaffected and stay bookable, since the flag records only the new "not a student" case.
 - A student's **name and birth year are now required**, marked in the form the same way a required registration question is and enforced on the server whichever way they were submitted. On signup the requirement applies only once the parent/guardian box is ticked, so registering for yourself is unaffected. A student block you have started filling in is now reported back to you rather than silently dropped when the name is missing — only a completely untouched spare block is still ignored.
 - Signup and the profile page now ask for a **birth year** rather than a full date of birth — a four-digit year between 1900 and the current year, with anything else discarded rather than stored. Students added before this change keep showing a birth year, derived from the date already on file; that old full date is then dropped the first time the record is saved, so the studio ends up holding only what it now asks for. No bulk purge runs, so a site wanting the remaining old dates gone should clear the `us_date_of_birth` user meta directly.
 - The interface now says **student** where it said "child" and **profile** where it said "family". The `[us_family]` page is headed **Your profile**, its form is **Add a student**, signup asks for a **Student's name**, and the wp-admin students list and student screen both label the relationship **Profile**. Two strings were reworded rather than swapped: the students list reads **Managed by _name_** (a bare "Student of _name_" would read as a teacher's pupil), and a managed account is described as a **managed student account** so it is not confused with the account holder. Internal names — database columns, request parameters, form field names, the `us_family` shortcode and the `us-scheduler/family` block — are unchanged, since they are contracts with existing installs and saved post content.