chore: upgrade React Native to 0.85.3

[txbrown]

Summary

• upgrade the library and example app from React Native 0.78.0 to 0.85.3, aligned with React 19.2.3
• apply the RN 0.85 Android and iOS template changes, including the new application entry points and toolchain versions
• move iOS native dependencies to RN 0.85 prebuilt artifacts, which include fmt 12.1.0 and compile under C++20 on Apple Clang 21
• keep the test/lint scopes limited to this repository rather than vendored Elementary sources

Validation

• yarn test --runInBand
• yarn typecheck
• yarn lint
• ./example/android/gradlew -p example/android :app:assembleDebug --no-daemon --console=plain -PreactNativeArchitectures=arm64-v8a
• npm run build:ios

Both native builds completed successfully on the local Xcode 26.5 / Android toolchain.