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/shared/ipc/contracts.ts

@@ -45,6 +45,8 @@ export const ipcChannels = {
     importZip: 'project.importZip',
     exportZip: 'project.exportZip',
     deleteProject: 'project.deleteProject',
+    importZipProgress: 'project.importZipProgress',
+    exportZipProgress: 'project.exportZipProgress',
   },
   windows: {
     openMultiWindow: 'windows.openMultiWindow',
@@ -73,6 +75,22 @@ export const ipcChannels = {
   },
 } as const;
 
+export type ZipProgressEvent = {
+  kind: 'import' | 'export';
+  stage: 'copy' | 'unzip' | 'zip' | 'done';
+  percent: number; // 0..100
+  detail?: string;
+};
+
+export type IpcEventMap = {
+  [ipcChannels.session.stateChanged]: { state: SessionState };
+  [ipcChannels.effects.stateChanged]: { state: EffectsState };
+  [ipcChannels.video.stateChanged]: { state: VideoPlaybackState };
+  [ipcChannels.license.statusChanged]: { snapshot: LicenseSnapshot };
+  [ipcChannels.project.importZipProgress]: ZipProgressEvent;
+  [ipcChannels.project.exportZipProgress]: ZipProgressEvent;
+};
+
 export type IpcInvokeMap = {
   [ipcChannels.app.quit]: {
     req: Record<string, never>;
@@ -237,7 +255,7 @@ export type SessionState = {
   currentSceneId: SceneId | null;
 };
 
-export type IpcEventMap = {
+export type LegacyIpcEventMap = {
   [ipcChannels.session.stateChanged]: { state: SessionState };
   [ipcChannels.effects.stateChanged]: { state: EffectsState };
   [ipcChannels.video.stateChanged]: { state: VideoPlaybackState };
@@ -249,6 +267,7 @@ export type ScenePatch = {
   description?: string;
   previewAssetId?: AssetId | null;
   previewAssetType?: 'image' | 'video' | null;
+  previewThumbAssetId?: AssetId | null;
   previewVideoAutostart?: boolean;
   previewRotationDeg?: 0 | 90 | 180 | 270;
   settings?: Partial<Scene['settings']>;