Feature/BT-7067 - Upgrade to React Native 0.81 and React 19

[BartoszLitwiniuk]

Summary:

Upgrades the @backtrace/react-native package (and tools) with its example app to React Native 0.81 and React 19.

Dependencies

• Upgrade react → 19.1.4, react-native (and tools) → ^0.81.6, @types/react → ^19.1.0
• Minimum Node bumped to >=20 in the example app.
• Add @react-native/babel-preset (package) and @react-native-community/cli
• Regenerates lockfiles, Podfile, and Android/iOS project config

React-native example app

• Upgrade gradle wrapper from 8.9 to 8.14
• Removing deprecated setting isHermesEnabled from MainApplication.java
• Babel: switch preset from deprecated module:metro-react-native-babel-preset
  to module:@react-native/babel-preset
• Source map script: resolve Metro's internal modules (bundleToString, baseJSBundle, CountingSet) by absolute path to bypass Node's exports enforcement in Metro ≥ 0.82
• DebuggerHelper: detect bridgeless (New Architecture) mode via the canonical RN$Bridgeless global instead of relying on NativeModules.UIManager, which is now unreliable.
• ErrorBoundary: guard against a nullable componentStack (React 19 type change).

[KishanPRao]

• Update title & description with what we've validated, what doesn't work, and what wasn't covered.
• Verify sourcemap upload (Metro 0.82) and new-arch bridge detection (both changed shipping code with no tests).

@melekr This goes to RN 0.81.6 + React 19 (a major bump), beyond what's in #376. Can you re-confirm you're good with these versions?

@BartoszLitwiniuk

1. New arch is on for iOS (RCTNewArchEnabled=true) but still off on Android (newArchEnabled=false). Is Android new arch just not flipped yet, or was there an issue?
2. Do we still need to support older React/RN (peerDeps are *), or is React 19 / RN 0.81 the new baseline?