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 -->

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

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

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