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/App/Tools.swift

@@ -0,0 +1,21 @@
+//
+//  Tools.swift
+//  AFFiNE
+//
+//  Created by qaq on 9/18/25.
+//
+
+import Foundation
+
+extension Optional {
+  func get(_ failure: String? = nil) throws -> Wrapped {
+    guard let self else {
+      if let failure {
+        throw NSError(domain: #function, code: -1, userInfo: [NSLocalizedDescriptionKey: failure])
+      } else {
+        throw NSError(domain: #function, code: -1)
+      }
+    }
+    return self
+  }
+}