Motion blur was a boolean switch (on/off). This changes it to a slider
from 0 (off) to 1 (full intensity), with 0.35 as the recommended sweet
spot per feedback on PR #207.
- EditorState/ProjectEditorState: motionBlurEnabled:bool → motionBlurAmount:number
- SettingsPanel: Switch → Slider (0–1, step 0.01); shows 'off' or value
- VideoPlayback/zoomTransform: scale blur by amount instead of boolean gate
- FrameRenderer/VideoExporter/GifExporter: propagate numeric amount
- projectPersistence: backward-compat loader (old true → 0.35, false → 0)
Siddharth