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.test.ts

@@ -42,6 +42,7 @@ void test('buildNextSceneCardById: does not change refs when irrelevant fields c
         connections: [],
         layout: { x: 0, y: 0 },
         previewAssetId: null,
+        previewThumbAssetId: null,
         previewAssetType: null,
         previewVideoAutostart: false,
         previewRotationDeg: 0,
@@ -79,6 +80,7 @@ void test('buildNextSceneCardById: changes card when title changes', () => {
         connections: [],
         layout: { x: 0, y: 0 },
         previewAssetId: null,
+        previewThumbAssetId: null,
         previewAssetType: null,
         previewVideoAutostart: false,
         previewRotationDeg: 0,