fix: retain original image titles when proxying previews

tests/e2e/specs/picture-title.spec.ts

@@ -0,0 +1,14 @@
+import { test, expect } from '@playwright/test';
+
+const fixtureBase = process.env.FIXTURE_BASE_URL || 'http://127.0.0.1:18080';
+
+for (const proxy of [false, true]) {
+  test(`image title retains the source filename with kkagent=${proxy}`, async ({ page }) => {
+    const source = `${fixtureBase}/sample.png`;
+    const encoded = encodeURIComponent(Buffer.from(source).toString('base64'));
+    const response = await page.goto(`/onlinePreview?url=${encoded}&kkagent=${proxy}`);
+
+    expect(response?.status()).toBe(200);
+    await expect(page.locator('.viewer-title')).toHaveText('sample.png');
+  });
+}