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/zipStore.legacyContract.test.ts

@@ -39,3 +39,9 @@ void test('zipStore: exportProjectZipToPath flushes saveNow for currently open p
   assert.match(src, /if \(this\.openProject\?\.id === projectId\)\s*\{\s*await this\.saveNow\(\);\s*\}/);
   assert.match(src, /await fs\.copyFile\(src, dest\)/);
 });
+
+void test('zipStore: normalizeScene defaults previewThumbAssetId for older projects', () => {
+  const src = fs.readFileSync(path.join(here, 'zipStore.ts'), 'utf8');
+  assert.match(src, /previewThumbAssetId/);
+  assert.match(src, /function normalizeScene\(/);
+});