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/optimizeImageImport.test.ts

@@ -0,0 +1,54 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import sharp from 'sharp';
+
+import { optimizeImageBufferVisuallyLossless } from './optimizeImageImport.lib.mjs';
+
+void test('optimizeImageBufferVisuallyLossless: preserves dimensions for opaque RGB', async () => {
+  const input = await sharp({
+    create: {
+      width: 400,
+      height: 300,
+      channels: 3,
+      background: { r: 10, g: 100, b: 200 },
+    },
+  })
+    .png({ compressionLevel: 0 })
+    .toBuffer();
+
+  const out = await optimizeImageBufferVisuallyLossless(input);
+  assert.equal(out.passthrough, false);
+  assert.ok(out.buffer.length > 0);
+  assert.ok(out.buffer.length < input.length);
+  const meta = await sharp(out.buffer).metadata();
+  assert.equal(meta.width, 400);
+  assert.equal(meta.height, 300);
+});
+
+void test('optimizeImageBufferVisuallyLossless: preserves dimensions for alpha', async () => {
+  const input = await sharp({
+    create: {
+      width: 200,
+      height: 200,
+      channels: 4,
+      background: { r: 255, g: 0, b: 0, alpha: 0.5 },
+    },
+  })
+    .png({ compressionLevel: 0 })
+    .toBuffer();
+
+  const out = await optimizeImageBufferVisuallyLossless(input);
+  assert.equal(out.passthrough, false);
+  assert.ok(out.mime === 'image/webp' || out.mime === 'image/png');
+  assert.ok(out.buffer.length < input.length);
+  const meta = await sharp(out.buffer).metadata();
+  assert.equal(meta.width, 200);
+  assert.equal(meta.height, 200);
+  assert.equal(meta.hasAlpha, true);
+});
+
+void test('optimizeImageBufferVisuallyLossless: non-image buffer is passthrough', async () => {
+  const out = await optimizeImageBufferVisuallyLossless(Buffer.from('not an image'));
+  assert.equal(out.passthrough, true);
+});