Add fly speed multiplier slider to model viewer

[michidk]

Summary

• Adds a Fly Speed multiplier slider (0.1x – 20x) to the model viewer camera settings panel, below the existing FOV slider
• Allows adjusting fly camera movement speed without code changes, matching the existing UI pattern (slider + label + spacer)

Changes

• ModelViewer_Camera.cs: Added flySpeedMultiplier field, applied to WASD movement and scroll-wheel vertical movement
• ModelViewer_Settings.cs: Added flySpeedMultiplier setting with SetFlySpeedMultiplier() (clamped 0.1–20x)
• ModelViewer_GUI.cs: Added flySpeedSlider field with initial sync and onValueChanged callback, following the FOV slider pattern
• ModelViewer.unity: Added Fly Speed slider UI element with proper spacer in the Camera settings panel