feat(badges): wire duplex field — hide badge back for single-sided templates

- Add duplex + style_href to properties_to_save so they're cached in IDB
- Derive show_badge_back: true when duplex is null/unset (safe default for
  existing templates) or 1; false when duplex=0 (single-sided, e.g. Axonius
  NYC using Zebra ZC10L PVC cards)
- Wrap entire badge_back section in {#if show_badge_back} so the DOM node
  is fully removed rather than just hidden — cleaner than a CSS class approach

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

src/lib/ae_events/ae_events__event_badge_template.ts

@@ -35,6 +35,8 @@ export const properties_to_save = [
     'show_qr_back',
     'layout',
     'style_filename',
+    'style_href',
+    'duplex',
     'enable',
     'hide',
     'priority',