Require a name and birth year for every student

Both fields are marked in their labels the same way a required registration
question is, and enforced on the server whichever form they arrive from:
GuardianService::createChild() and updateChild() now refuse a blank name or
an unusable birth year, and the signup form checks the same rule up front,
before it creates a single user, so a bad block never leaves a
half-registered family behind. normaliseBirthYear() became public and static
so both paths share one definition of what a usable year is.

The signup form cannot lean on the browser here. Its child blocks are hidden
until the parent/guardian box is ticked, and a `required` field inside a
hidden container makes the whole form unsubmittable with no control the user
can reach to fix — the same trap the guardian's own question panel already
sidesteps by disabling rather than hiding. So register.js puts `required` on
and takes it off along with the block itself, and the server is what makes
the rule hold with JavaScript off. The profile screen has no such problem:
its forms are always visible, so the attribute is static there.

One behaviour change beyond the requirement: a child block with anything
typed into it is now reported back instead of dropped. Previously any block
without a name was silently discarded, which would now mean losing a birth
year the guardian had filled in. A wholly untouched spare block — the one
the form always renders for "add another" — is still ignored.

Verified the required-toggling in a headless browser: unticked submits,
ticked blocks an empty block, a cloned block inherits the requirement, and
re-unticking leaves nothing behind to block a non-guardian signup.

Closes #148

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

CHANGELOG.md

@@ -14,6 +14,7 @@ each change under the current top section as you work.
 ## [1.3.1]
 
 ### Changed
+- 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.