Let the account holder edit their own profile details

The Profile block is headed "Your profile", but the one person on it you
could not change was yourself: your name, your birth year, and whether you
take lessons yourself were fixed at whatever signup recorded, and correcting
any of them meant asking a studio admin.

A "Your details" section now opens the page, saved through the same
nonce-checked template_redirect post/redirect/get path the child rows use:

- Your name, written to display_name and nickname together, for the reason
  updateChild() does — UserName reads the nickname first, and leaving it
  behind would put the account's email address back on every screen that
  names a person.
- "I take lessons myself", the positive of us_guardian_only. This makes good
  on the claim already in bookableStudents() and the feature doc that a
  guardian-only account can put itself right from the profile page.
- Your birth year, held to the same normaliseBirthYear() rule as every other
  student.

The email is shown but not editable: it is the account's user_login as well
as its address, so changing it stays a studio-side job.

The birth-year field deliberately carries no `required` attribute. It is
asked of a student only, and this page loads no JavaScript, so a
browser-enforced `required` would leave a guardian who books solely for
other people unable to submit the form at all; handleSelf() enforces it
against the checkbox instead. Unticking the box does not clear a stored
birth year — it says who books, not "forget what is on file".

Closes #165

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

tests/Unit/BlockPreviewTest.php

@@ -74,6 +74,23 @@ class BlockPreviewTest extends TestCase
         self::assertStringContainsString('us-editor-note', $html);
     }
 
+    /**
+     * The preview is what someone placing the block styles against, so it has to
+     * show both halves of the page — your own details as well as your students'.
+     */
+    public function testFamilyPreviewShowsTheAccountHoldersDetailsAndTheirStudents(): void
+    {
+        $html = BlockPreview::family();
+
+        self::assertStringContainsString('class="us-family-self"', $html);
+        self::assertStringContainsString('id="us-own-name"', $html);
+        self::assertStringContainsString('id="us-own-birth-year"', $html);
+        self::assertStringContainsString('I take lessons myself', $html);
+        self::assertStringContainsString('class="us-family-list"', $html);
+        self::assertStringContainsString('class="us-family-add"', $html);
+        self::assertStringContainsString('us-editor-note', $html);
+    }
+
     public function testRegistrationPreviewShowsADisabledSampleForm(): void
     {
         $html = BlockPreview::registration();