chore: fix building the app (#13606)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- New Features
  - Built-in Then-style DSL for fluent configuration.
- Centralized theming via a new resources library exposing standardized
colors and icons for SwiftUI and UIKit.

- Refactor
  - Migrated color and icon accessors to the new resources provider.
  - Removed redundant imports and streamlined UI configuration.

- Dependencies
  - Updated MarkdownView to 3.4.2.
- Removed the Then third-party dependency; updated package sources;
added resources package and assets.

- Documentation
  - Added iOS Swift code style and architecture guidelines.

- Chores
  - Updated Xcode project format and repository ignore rules.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/apps/ios/App/Packages/AffineResources/.gitignore

@@ -0,0 +1,8 @@
+.DS_Store
+/.build
+/Packages
+xcuserdata/
+DerivedData/
+.swiftpm/configuration/registries.json
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+.netrc