Replace custom AppTypography with native iOS font sizes

Removed the custom AppTypography system and migrated all font references
to use native SwiftUI Dynamic Type sizes. This enables automatic text
scaling based on user preferences and ensures consistency with iOS
design standards.

Changes:
- Removed AppTypography struct from DesignSystem.swift
- Updated PrimaryButtonStyle and SecondaryButtonStyle to use .headline
- Replaced all typography references throughout the app with native fonts:
  - displayLarge → .largeTitle.weight(.bold)
  - headlineSmall → .title3.weight(.semibold)
  - titleMedium → .title3.weight(.semibold)
  - bodyMedium → .body
  - labelMedium → .footnote.weight(.medium)
  - And many more across 18 files

Benefits:
- Supports Dynamic Type for accessibility
- Reduces custom code maintenance
- Aligns with iOS design guidelines

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

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

iosApp/iosApp/Contractor/ContractorFormSheet.swift

@@ -190,7 +190,7 @@ struct ContractorFormSheet: View {
                                     .frame(width: 20)
 
                                 Text("Private Notes")
-                                    .font(AppTypography.labelMedium)
+                                    .font(.footnote.weight(.medium))
                                     .foregroundColor(AppColors.textSecondary)
                             }
 
@@ -208,7 +208,7 @@ struct ContractorFormSheet: View {
                                 Image(systemName: "star.fill")
                                     .foregroundColor(isFavorite ? AppColors.warning : AppColors.textSecondary)
                                 Text("Mark as Favorite")
-                                    .font(AppTypography.bodyMedium)
+                                    .font(.body)
                                     .foregroundColor(AppColors.textPrimary)
                             }
                         }
@@ -219,7 +219,7 @@ struct ContractorFormSheet: View {
                         // Error Message
                         if let error = viewModel.errorMessage {
                             Text(error)
-                                .font(AppTypography.bodySmall)
+                                .font(.callout)
                                 .foregroundColor(AppColors.error)
                                 .padding(AppSpacing.sm)
                                 .frame(maxWidth: .infinity)
@@ -360,7 +360,7 @@ struct SectionHeader: View {
     var body: some View {
         HStack {
             Text(title)
-                .font(AppTypography.titleSmall)
+                .font(.headline)
                 .foregroundColor(AppColors.textPrimary)
             Spacer()
         }
@@ -385,12 +385,12 @@ struct FormTextField: View {
                         .frame(width: 20)
 
                     Text(title)
-                        .font(AppTypography.labelMedium)
+                        .font(.footnote.weight(.medium))
                         .foregroundColor(AppColors.textSecondary)
                 }
             } else {
                 Text(title)
-                    .font(AppTypography.labelMedium)
+                    .font(.footnote.weight(.medium))
                     .foregroundColor(AppColors.textSecondary)
             }