feat(ios): [IAP] Paywall Initial Commit (#13609)

Requires https://github.com/toeverything/AFFiNE/pull/13606 to be merged.

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

- New Features
- Introduced an in-app Paywall with Pro, AI, and Believer plans, feature
previews, paging dots, and selectable pricing options.
- Added purchase and restore actions, plus a unified, polished UI using
new color/icon resources.

- Documentation
  - Added Swift Code Style Guidelines.

- Chores
- Updated dependencies (including MarkdownView 3.4.2), added new
resource packages, and removed an unused dependency.
  - Raised iOS deployment target to 16.5 and refreshed project settings.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: DarkSky <25152247+darkskygit@users.noreply.github.com>

packages/frontend/apps/ios/App/Packages/AffinePaywall/Package.swift

@@ -0,0 +1,27 @@
+// swift-tools-version: 6.2
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+  name: "AffinePaywall",
+  platforms: [
+    .iOS(.v16),
+    .macOS(.v14), // just for build so LLM can verify their code
+  ],
+  products: [
+    .library(
+      name: "AffinePaywall",
+      targets: ["AffinePaywall"]
+    ),
+  ],
+  dependencies: [
+    .package(path: "../AffineResources"),
+  ],
+  targets: [
+    .target(
+      name: "AffinePaywall",
+      dependencies: ["AffineResources"],
+    ),
+  ]
+)