fix(ios): allow transparent safe area views

[huytdps13400]

Fixes #197

Summary

• mark the legacy iOS SafeAreaView as non-opaque during initialization
• apply the same invariant to the Fabric component view
• keep layout, background-color handling, and the public JavaScript API unchanged

UIView defaults to opaque. Apple documents that a fully or partially transparent view must set opaque to false; otherwise rendering is unpredictable. SafeAreaView accepts transparent backgrounds, so both native implementations now opt into normal compositing.

Apple reference: https://developer.apple.com/documentation/uikit/uiview/isopaque

Test Plan

• RED on current main: instrumented the example app on an iPhone 17 Pro simulator (iOS 26.5, Xcode 26.6, React Native 0.85.3); Fabric SafeAreaView initialized with opaque=YES
• GREEN after the change: the same runtime probe reported opaque=NO, and the red/blue background remained visible through the top safe-area inset
• yarn format:clang:check
• yarn validate:typescript
• yarn validate:eslint (0 errors; 3 pre-existing React Native deep-import warnings)
• yarn validate:jest --runInBand (4 suites, 21 tests, 11 snapshots)
• xcodebuild -workspace example/ios/RNSACExample.xcworkspace -scheme safe-area-example -configuration Debug -destination platform=iOS Simulator,id=D463F2F5-3B09-4E46-8919-C69E61AB557E CODE_SIGNING_ALLOWED=NO build