feat: persist empty-state notice dismissal via user_meta

The "No active/completed notes found" notice was already dismissible
per-page-load (WP core's X button hides it), but pressing X only
cleared it for the current view — it returned on the next refresh.
The dismissal is now persisted to user_meta per-user-per-list-type,
so once closed it stays closed across page loads until the flag is
reset.

MECHANICS
- inc/wp-notes-display.php now checks get_user_meta(uid,
  'wp_notes_dismissed_empty_<type>') before rendering and skips the
  notice render entirely when set.
- New AJAX handler wp_ajax_wp_notes_dismiss_empty validates a nonce
  and the edit_posts capability, then writes the flag via
  update_user_meta(). type=active|completed; anything else 400s.
- An inline jQuery handler in wp_notes_add_inline_scripts() listens
  for clicks on .wp-notes-empty .notice-dismiss (WP core's auto-
  injected X button) and fires the AJAX call. The visual hide is
  still WP core's job.
- The notice element carries data-wp-notes-empty-type and a fresh
  per-render data-wp-notes-nonce for the round trip.

RESET
The flag is per-user-meta keyed wp_notes_dismissed_empty_active /
wp_notes_dismissed_empty_completed. wp_delete_user removes them
automatically. A UI button to reset dismissed notices is not built
yet — flagged in the changelog as a future enhancement.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

inc/wp-notes-display.php

@@ -22,14 +22,24 @@ function wp_notes_display_notes($type = 'active') {
     $section_title = ($type === 'active') ? 'Active Notes' : 'Completed Notes';
 
     if (empty($notes)) {
-        /* is-dismissible adds an X button via WP core's common.js so
-           users can clear the empty-state message for the current view.
-           Dismissal is per-page-load (the message reappears on next
-           refresh if the list is still empty) — persistence across
-           reloads would need user_meta tracking and isn't built yet. */
-        echo '<div class="wp-notes-empty notice notice-info is-dismissible">';
-        echo '<p>' . esc_html__('No ' . ($type === 'active' ? 'active' : 'completed') . ' notes found.', 'a-wp-notes') . '</p>';
-        echo '</div>';
+        /* Per-user persistent dismissal: if the current user has
+           dismissed the empty-state notice for this list type before,
+           we skip rendering it entirely. The flag stays set until they
+           explicitly reset it (or until we extend this to auto-clear
+           when a note is created — Option B for a future iteration).
+           Flag key: wp_notes_dismissed_empty_active / _completed. */
+        $dismiss_meta_key = 'wp_notes_dismissed_empty_' . $type;
+        if (get_user_meta(get_current_user_id(), $dismiss_meta_key, true)) {
+            return;
+        }
+
+        $nonce = wp_create_nonce('wp_notes_dismiss_empty');
+        printf(
+            '<div class="wp-notes-empty notice notice-info is-dismissible" data-wp-notes-empty-type="%1$s" data-wp-notes-nonce="%2$s"><p>%3$s</p></div>',
+            esc_attr($type),
+            esc_attr($nonce),
+            esc_html__('No ' . ($type === 'active' ? 'active' : 'completed') . ' notes found.', 'a-wp-notes')
+        );
         return;
     }