Group class term dates, single-class embed mode, and offering editing

Group class offerings now carry real dates: the add/edit form takes a
start date plus a sessions control (one-off, or weekly for N sessions;
the end date is computed as start + (N-1) weeks via
Offering::weeklyTermEnd). Dates are validated strictly (Y-m-d) and shown
in the offerings list and on the student-facing class card, including
the weekly session count.

[us_group_classes offering="<id>"] (block attribute offeringId, chosen
from a dropdown of active classes fetched from the public offerings
endpoint) restricts the page to a single class so the enrolment flow can
be embedded on a page dedicated to that class; a pinned class that is no
longer offered reports itself closed instead of falling back to the
catalog.

Offerings are now editable from the admin screen: an Edit button
prefills the shared add/edit form and saving posts usc_action=update.
Updates always preserve the original owner and currency, and non-admin
instructors can only load and update their own offerings. The form also
gains the previously missing description field and an Active toggle (the
admin-UI counterpart of the REST is_active flag) so an edit cannot wipe
data the form never collected.

Closes #59

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

src/BlockRegistrar.php

@@ -42,7 +42,7 @@ class BlockRegistrar {
 		wp_register_script(
 			self::SCRIPT_HANDLE,
 			USC_PLUGIN_URL . 'assets/js/blocks.js',
-			[ 'wp-blocks', 'wp-element', 'wp-block-editor', 'wp-components', 'wp-data', 'wp-core-data', 'wp-server-side-render', 'wp-i18n' ],
+			[ 'wp-blocks', 'wp-element', 'wp-block-editor', 'wp-components', 'wp-data', 'wp-core-data', 'wp-server-side-render', 'wp-i18n', 'wp-api-fetch' ],
 			USC_VERSION,
 			true
 		);
@@ -108,7 +108,12 @@ class BlockRegistrar {
 			],
 			'us-scheduler/group-classes'    => [
 				'render'     => [ $this, 'renderGroupClasses' ],
-				'attributes' => [],
+				'attributes' => [
+					'offeringId' => [
+						'type'    => 'number',
+						'default' => 0,
+					],
+				],
 			],
 		];
 	}