Add an account block showing who is signed in

[us_account], or the Account block: the signed-in visitor's name, their
email, a Sign out link, and — only when the account books for someone
besides itself — the students it books for. A parent's first question on
seeing "signed in as Grace" is whether this is the account their children's
lessons are on.

Two decisions worth naming.

Signed out with no login page chosen, the block renders nothing. Its whole
subject is the person signed in, which a stranger is not, and a bare "you
are not signed in" in a site header is noise with no way to act on it. With
a login page chosen it offers a Sign in link instead. The editor preview is
populated regardless, so the block is never an invisible box to the person
placing it.

Signing out returns to the chosen login page, or to the current page when
there is none. A block meant for a header should not also navigate someone
somewhere when they use it; the login page wins when configured, because the
page they were on may well be members-only.

The name comes from UserName::format(), so the block never exposes a
username the way display_name can.

Also brings docs/features/editor-blocks.md back in step: it still described
"four shortcodes" and had never listed the family block.

Closes #142

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

assets/css/frontend.css

@@ -525,6 +525,30 @@
     }
 }
 
+/*
+ * The account panel: who is signed in, and the way out. Sized to sit in a
+ * header or sidebar, so the rules stay minimal and inherit the theme's type —
+ * a block that lands in a site header should look like it belongs there.
+ */
+.us-account p {
+    margin: 0 0 4px;
+}
+
+.us-account-name {
+    font-weight: 600;
+}
+
+.us-account-email,
+.us-account-students {
+    display: block;
+    font-size: 0.9em;
+    opacity: 0.75;
+}
+
+.us-account-actions {
+    margin-top: 8px;
+}
+
 /* Shown only in block-editor previews (see BlockPreview). */
 .us-editor-note {
     font-size: 0.85em;