fix(web-app, admin): Angular 21+ modernizing for the event form preview dialog

[ktsorensenWX]

Summary

• Converts AdminEventFormPreviewComponent and its child AdminEventFormPreviewDialogComponent to standalone components — converted together since the parent opens the dialog directly via MatDialog.open().
• AdminEventFormPreviewDialogComponent imports ObservationEditModule (not the component) to bring in , which is still NgModule-based.
• AdminEventFormPreviewComponent needs no imports at all — its template has no directives, and the dialog reference it uses is a TS-level call, not a template selector.

Testing

• Fix stale declarations → imports in both components' .spec.ts files
• Update admin-event-form.module.ts (move both to imports, keep in exports — event-details.component.html depends on the re-export)
• Full npm run test-headless --prefix web-app pass
• Manual check: open an event form, trigger the preview dialog, confirm it renders and closes correctly