Migrate iOS app to system colors and improve UI/UX

- Remove AppColors struct, migrate to iOS system colors throughout
- Redesign ContractorFormSheet to use native SwiftUI Form components
- Add color-coded icons to contractor form sections
- Improve dark mode contrast for task cards
- Add background colors to document detail fields
- Fix text alignment issues in ContractorDetailView
- Make task completion lists expandable/collapsible by default
- Clear app badge on launch and when app becomes active
- Update button styling with proper gradients and shadows
- Improve form field focus states and accessibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

iosApp/iosApp/Task/CompleteTaskView.swift

@@ -393,7 +393,7 @@ struct ContractorPickerView: View {
                         Spacer()
                         if selectedContractor == nil {
                             Image(systemName: "checkmark")
-                                .foregroundStyle(AppColors.primary)
+                                .foregroundStyle(.blue)
                         }
                     }
                 }
@@ -441,7 +441,7 @@ struct ContractorPickerView: View {
 
                                 if selectedContractor?.id == contractor.id {
                                     Image(systemName: "checkmark")
-                                        .foregroundStyle(AppColors.primary)
+                                        .foregroundStyle(.blue)
                                 }
                             }
                         }