fix(frontend): import InboundFormModal.css so layout classes apply (B12)

The file InboundFormModal.css existed but was never imported, so every
class in it had no effect — including:

- .vless-auth-state — the "Selected: <auth>" caption next to the X25519/
  ML-KEM/Clear button row stayed inline next to Clear instead of
  display:block beneath the row
- .advanced-shell / .advanced-panel — the Advanced tab's header / panel
  framing was missing
- .advanced-editor-meta — the per-section help text under each Advanced
  sub-tab had no spacing
- .wg-peer — wireguard peer rows had no top margin

Add a side-effect import of the CSS file at the top of the modal. No
other change needed; the legacy modal must have either imported it or
had a global import that the new modal didn't inherit.

frontend/src/pages/inbounds/InboundFormModal.tsx

@@ -69,6 +69,7 @@ import FinalMaskForm from '@/components/FinalMaskForm';
 import HeaderMapEditor from '@/components/HeaderMapEditor';
 import InputAddon from '@/components/InputAddon';
 import JsonEditor from '@/components/JsonEditor';
+import './InboundFormModal.css';
 import type { FormInstance } from 'antd';
 import type { NamePath } from 'antd/es/form/interface';