Update RestoreSystemCollection handler to support form-encoded requests from HTMX:
- Add 'form' struct tags to CollectionName and ResetType fields to enable binding
from both JSON payloads and form submissions (required for HTMX compatibility)
- Add conditional HTMX redirect handling that sets HX-Redirect header when
the request originates from HTMX, directing users to /collections after
successful restoration
This change enables the system collection restore functionality to work seamlessly
with HTMX-based modal forms, improving the user experience by providing proper
navigation after the restore operation completes without requiring JavaScript
redirect logic.