feat(project): optimize image imports and converter

- Optimize imported scene preview images (smart WebP/JPEG/PNG, preserve alpha, keep pixel size)

- Update converter to re-encode existing image assets with same algorithm

- Improve import/export progress overlay and reduce presentation slide stutter

Made-with: Cursor

app/renderer/editor/graph/sceneCardById.ts

@@ -38,6 +38,7 @@ export function buildNextSceneCardById(
     if (
       prevCard?.title === s.title &&
       prevCard.previewAssetId === s.previewAssetId &&
+      prevCard.previewThumbAssetId === s.previewThumbAssetId &&
       prevCard.previewAssetType === s.previewAssetType &&
       prevCard.previewVideoAutostart === s.previewVideoAutostart &&
       prevCard.previewRotationDeg === s.previewRotationDeg &&
@@ -49,6 +50,7 @@ export function buildNextSceneCardById(
       nextMap[id] = {
         title: s.title,
         previewAssetId: s.previewAssetId,
+        previewThumbAssetId: s.previewThumbAssetId,
         previewAssetType: s.previewAssetType,
         previewVideoAutostart: s.previewVideoAutostart,
         previewRotationDeg: s.previewRotationDeg,