feat(editor): show scene preview immediately and optimize in background

Return control after copying the original asset, then run image optimization
and thumbnail generation in the background with IPC progress updates.
Block duplicate preview uploads and scene creation while requests are in flight.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/shared/ipc/contracts.ts

@@ -55,6 +55,7 @@ export const ipcChannels = {
     deleteProject: 'project.deleteProject',
     importZipProgress: 'project.importZipProgress',
     exportZipProgress: 'project.exportZipProgress',
+    scenePreviewImportProgress: 'project.scenePreviewImportProgress',
   },
   windows: {
     openMultiWindow: 'windows.openMultiWindow',
@@ -97,6 +98,14 @@ export type ZipProgressEvent = {
   detail?: string;
 };
 
+export type ScenePreviewImportEvent = {
+  sceneId: SceneId;
+  assetId: AssetId;
+  phase: 'queued' | 'optimizing' | 'thumbnail' | 'done' | 'error';
+  project?: Project;
+  message?: string;
+};
+
 export type UpdaterCheckResponse =
   | { outcome: 'not_packaged' }
   | { outcome: 'no_license' }
@@ -131,6 +140,7 @@ export type IpcEventMap = {
   [ipcChannels.windows.multiWindowStateChanged]: { open: boolean };
   [ipcChannels.project.importZipProgress]: ZipProgressEvent;
   [ipcChannels.project.exportZipProgress]: ZipProgressEvent;
+  [ipcChannels.project.scenePreviewImportProgress]: ScenePreviewImportEvent;
   [ipcChannels.updater.progress]: UpdaterProgressEvent;
 };
 
@@ -205,7 +215,7 @@ export type IpcInvokeMap = {
   };
   [ipcChannels.project.importScenePreview]: {
     req: { sceneId: SceneId };
-    res: { project: Project };
+    res: { project: Project; assetId: AssetId | null; background: boolean };
   };
   [ipcChannels.project.clearScenePreview]: {
     req: { sceneId: SceneId };