Exclude archived/deleted/spammed sites from user notification settings picker

[Copilot]

On multisite networks, the user notification settings picker displayed all Falcon-enabled sites regardless of their status, causing archived, deleted, and spammed sites to clutter the UI.

Changes

• filter_inactive_sites( $sites ) — new protected static helper on Falcon_Manager that intersects a list of enabled site IDs against available_sites() (which already queries wp_get_sites() with archived=false, deleted=false, spam=false)
• user_profile_fields() — applies filter_inactive_sites() before passing sites to the network profile fields action, so inactive sites never render in the picker
• save_profile_settings() — applies the same filter before dispatching the save action, keeping display and persistence consistent

protected static function filter_inactive_sites( $sites ) {
    $available_site_ids = wp_list_pluck( self::available_sites(), 'blog_id' );
    return array_values( array_intersect( $sites, $available_site_ids ) );
}

The falcon_enabled_sites option itself is unchanged — the filtering happens at runtime so it self-corrects as site statuses change without requiring any data migration.