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>

composeApp/src/commonMain/kotlin/com/example/mycrib/ui/screens/DocumentsScreen.kt

@@ -77,7 +77,7 @@ fun DocumentsScreen(
                                 Icon(
                                     if (showActiveOnly) Icons.Default.CheckCircle else Icons.Default.CheckCircleOutline,
                                     "Filter active",
-                                    tint = if (showActiveOnly) Color(0xFF10B981) else LocalContentColor.current
+                                    tint = if (showActiveOnly) MaterialTheme.colorScheme.secondary else LocalContentColor.current
                                 )
                             }
                         }
@@ -89,7 +89,7 @@ fun DocumentsScreen(
                                     Icons.Default.FilterList,
                                     "Filters",
                                     tint = if (selectedCategory != null || selectedDocType != null)
-                                        Color(0xFF3B82F6) else LocalContentColor.current
+                                        MaterialTheme.colorScheme.primary else LocalContentColor.current
                                 )
                             }