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]>

src/BlockPreview.php

@@ -15,6 +15,12 @@ namespace Unsupervised\Schedular;
  */
 class BlockPreview {
 
+	/**
+	 * The marker a required field's label carries, matching the one
+	 * {@see Registration\QuestionField::render()} puts on a required question.
+	 */
+	private const REQUIRED_MARK = ' <span class="us-required" aria-hidden="true">*</span>';
+
 	/**
 	 * Sample booking page.
 	 *
@@ -186,8 +192,8 @@ class BlockPreview {
 		}
 
 		$add = sprintf(
-			'<h4>%s</h4><p><label for="us-child-name">%s</label><input type="text" id="us-child-name"></p>'
-				. '<p><label for="us-child-birth-year">%s</label><input type="number" id="us-child-birth-year" placeholder="YYYY"></p>'
+			'<h4>%s</h4><p><label for="us-child-name">%s' . self::REQUIRED_MARK . '</label><input type="text" id="us-child-name"></p>'
+				. '<p><label for="us-child-birth-year">%s' . self::REQUIRED_MARK . '</label><input type="number" id="us-child-birth-year" placeholder="YYYY"></p>'
 				. '<p><button type="button" disabled>%s</button></p>',
 			esc_html__( 'Add a student', 'unsupervised-schedular' ),
 			esc_html__( 'Name', 'unsupervised-schedular' ),