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/main/project/assetPrune.ts

@@ -9,6 +9,7 @@ export function collectReferencedAssetIds(p: Project): Set<AssetId> {
   const refs = new Set<AssetId>();
   for (const sc of Object.values(p.scenes)) {
     if (sc.previewAssetId) refs.add(sc.previewAssetId);
+    if (sc.previewThumbAssetId) refs.add(sc.previewThumbAssetId);
     for (const vid of sc.media.videos) refs.add(vid);
     for (const au of sc.media.audios) refs.add(au.assetId);
   }