refactor(mobile): modernize platform detection APIs with userAgentData

[guesung]

Summary

Modernize @react-simplikit/mobile platform detection utilities (isIOS, isAndroid) to use the latest browser APIs, referencing Adobe React Spectrum's platform.ts as a proven implementation.

Motivation

• navigator.platform is unreliable and not recommended — already replaced with UA string check in isIOS
• navigator.userAgentData is the modern replacement (MDN) and provides more structured, reliable detection
• React Spectrum uses userAgentData with fallback to legacy APIs — a pattern worth adopting

Proposed Changes

1. Use navigator.userAgentData as primary detection

// Current: regex on raw UA string
/Android/i.test(navigator.userAgent)

// Proposed: userAgentData first, UA string fallback
function testUserAgent(re: RegExp): boolean {
  if (typeof window === 'undefined') return false;
  const brands = navigator.userAgentData?.brands;
  return brands
    ? brands.some(b => re.test(b.brand))
    : re.test(navigator.userAgent);
}

2. Expand platform detection coverage (optional)

Consider adding utilities already proven useful in React Spectrum:

Utility	Purpose
isMac()	macOS detection
isIPhone()	iPhone-specific (vs iPad)
isIPad()	iPad-specific
isAppleDevice()	Any Apple device
isWebKit()	WebKit engine detection
isChrome()	Chrome detection
isFirefox()	Firefox detection

Reference

• React Spectrum platform.ts — full source
  • testUserAgent / testPlatform — userAgentData with legacy fallback
  • isIPad — iPadOS 13+ detection via isMac() && maxTouchPoints > 1

Checklist

• Add testUserAgent / testPlatform internal helpers with userAgentData support
• Migrate isIOS and isAndroid to use new helpers
• Decide which additional detection utilities to add
• Add/update tests and SSR safety tests
• 100% coverage maintained