Add touch event support for iOS/mobile platforms

[doxle]

🎯 Working Touch Events for Blitz + Dioxus

This PR adds complete touch event support, tested and working on physical iPhone device.

✅ What works:

• Touch events (start/move/end) detected in Blitz
• Dioxus ontouchstart, ontouchmove, ontouchend handlers work
• No crashes - fixed missing TouchData conversion
• Touch coordinates properly tracked

🔧 Key fixes:

• Added EmptyTouchData struct implementing HasTouchData
• Fixed convert_touch_data() panic in mini-dxn
• Map touch events to mouse events for Dioxus compatibility

📱 Tested on:

• Physical iPhone device with real touch input
• Simple Dioxus app with touch event handlers

Touch events now work end-to-end in Blitz! 🚀