feat: add duration property to VideoPlayerState

[kdroidFilter]

Summary

• Add duration: Double property to VideoPlayerState interface, exposing the total media duration in seconds
• Implement across all platforms: Android, iOS, Web, JVM (Linux, Mac, Windows)
• Follows the same pattern as the existing currentTime property
• Keep positionText and durationText as-is for convenience

Closes #94

Test plan

• Verify duration returns correct value in seconds on Android
• Verify duration returns correct value on JVM (Linux/Mac/Windows)
• Verify duration returns correct value on iOS
• Verify duration returns correct value on Web
• Verify duration is 0.0 when no media is loaded
• Verify positionText and durationText still work as before