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]>

assets/css/frontend.css

@@ -521,6 +521,32 @@
 }
 
 /* The guardian's manage-children screen ([us_family]). */
+
+/*
+ * The account holder's own details, set off from the students below so the two
+ * halves of the page do not read as one long form.
+ */
+.us-family-self {
+    margin-bottom: 24px;
+    padding-bottom: 16px;
+    border-bottom: 1px solid #eee;
+}
+
+.us-family-self-email span {
+    font-weight: 600;
+}
+
+/*
+ * Guidance under a control, not a label: it explains when a field matters
+ * rather than naming it, so it is sized down and reads after the input.
+ */
+.us-field-hint {
+    display: block;
+    margin-top: 4px;
+    font-size: 0.9em;
+    opacity: 0.75;
+}
+
 .us-family-list {
     margin: 0 0 20px;
     padding: 0;