chore(emulation): add platform enum for cross-platform header generation

[danielhe4rt]

Summary

• Add Platform enum (MacOS, Windows, Linux, Android, iOS) with:
  • secChUaPlatform() → "macOS", "Windows", etc.
  • isMobile() → true for Android/iOS
  • userAgentPlatform() → platform-specific UA string
  • random() → random platform selection
• Add ClientBuilder::platform(Platform) that rewrites profile headers at build time:
  • User-Agent → platform-specific (Macintosh → Windows NT → X11 Linux → Android → iPhone)
  • sec-ch-ua-platform → matching platform string
  • sec-ch-ua-mobile → ?1 for mobile, ?0 for desktop
• Add Browser::random() for random browser selection

Usage

$client = Client::builder()
    ->emulation(Browser::Chrome145)
    ->platform(Platform::Windows)  // Windows UA + headers
    ->build();

// Random browser + platform
$client = Client::builder()
    ->emulation(Browser::random())
    ->platform(Platform::random())
    ->build();

Test plan

• Platform enum: 5 cases, correct backed values, mobile detection, UA strings
• ClientBuilder: Windows platform changes UA to "Windows NT 10.0", Android sets mobile=?1
• Platform doesn't affect profiles without sec-ch-ua headers
• Browser::random() returns valid Browser case
• PHPStan level max: 0 errors