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/scenePreviewThumbnail.test.ts

@@ -0,0 +1,35 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+import test from 'node:test';
+
+import sharp from 'sharp';
+
+import { generateScenePreviewThumbnailBytes, SCENE_PREVIEW_THUMB_MAX_PX } from './scenePreviewThumbnail';
+
+void test('generateScenePreviewThumbnailBytes: image scales to max edge', async () => {
+  const tmp = await fs.mkdtemp(path.join(os.tmpdir(), 'dnd-thumb-test-'));
+  const src = path.join(tmp, 'wide.png');
+  await sharp({
+    create: {
+      width: 800,
+      height: 400,
+      channels: 3,
+      background: { r: 200, g: 40, b: 40 },
+    },
+  })
+    .png()
+    .toFile(src);
+
+  const buf = await generateScenePreviewThumbnailBytes(src, 'image');
+  assert.ok(buf !== null);
+  assert.ok(buf.length > 0);
+  const meta = await sharp(buf).metadata();
+  assert.equal(typeof meta.width, 'number');
+  assert.equal(typeof meta.height, 'number');
+  assert.ok(meta.width > 0 && meta.height > 0);
+  assert.ok(Math.max(meta.width, meta.height) <= SCENE_PREVIEW_THUMB_MAX_PX);
+
+  await fs.rm(tmp, { recursive: true, force: true });
+});