chore: Update sample app to ease testing

[jamesnrokt]

Summary

• While debugging a partner issue I needed to use a custom layout, this PR adds that via a text input
• Additionally improves usability of the sample app with better view handling around keyboards and scrolling

Testing Plan

Tested locally and verified with live stream

Master Issue

Closes N/A

[Copilot]

Pull request overview

This PR enhances the sample app's usability for testing custom layouts by adding a text input field for custom placement identifiers and improving the UI/UX with better keyboard handling and visual design updates. The changes also include dependency updates via yarn.lock and streamlined installation instructions.

Changes:

• Added custom layout identifier text input with improved UI styling
• Implemented keyboard-aware scrolling with SafeAreaView and KeyboardAvoidingView
• Updated iOS configuration and simplified onboarding documentation

Reviewed changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
sample/yarn.lock	Updated JavaScript dependencies to latest versions
sample/package.json	Added postinstall script to automatically install iOS pods
sample/index.js	Added custom identifier input, improved UI with better styling and keyboard handling
sample/ios/MParticleSample/AppDelegate.mm	Updated mParticle initialization with debug logging and production environment
sample/ios/MParticleSample.xcodeproj/project.pbxproj	Added development team ID and updated Xcode build configuration
ONBOARDING.md	Simplified setup instructions by removing manual pod install step

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