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/Package.swift

@@ -0,0 +1,23 @@
+// swift-tools-version: 5.9
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+  name: "AffineResources",
+  products: [
+    .library(
+      name: "AffineResources",
+      targets: ["AffineResources"]
+    ),
+  ],
+  targets: [
+    .target(
+      name: "AffineResources",
+      resources: [
+        .process("Resources/Icons.xcassets"),
+        .process("Resources/Colors.xcassets"),
+      ]
+    ),
+  ]
+)