feat(ui): add sport backgrounds to share cards, achievement filtering, and wizard validation

- Add ShareCardSportBackground with floating sport icons for share cards
- Share cards now show sport-specific backgrounds (single or multiple sports)
- Achievement collection share respects sport filter selection
- Add ability to share individual achievements from detail sheet
- Trip wizard ReviewStep highlights missing required fields in red
- Add FieldValidation model to TripWizardViewModel

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Features/Trip/Views/Wizard/TripWizardView.swift

@@ -72,6 +72,7 @@ struct TripWizardView: View {
                                 mustStopLocations: viewModel.mustStopLocations,
                                 isPlanning: viewModel.isPlanning,
                                 canPlanTrip: viewModel.canPlanTrip,
+                                fieldValidation: viewModel.fieldValidation,
                                 onPlan: { Task { await planTrip() } }
                             )
                         }