chore(core): pdf viewer is enabled by default (#12207)

Related to: [BS-3101](https://linear.app/affine-design/issue/BS-3101/pdf-embed-模式的选中框选-和点开看详情有比较大的问题)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - PDF embed preview is now enabled by default for all users, regardless of workspace sharing mode.

- **Bug Fixes**
  - Improved reliability of PDF viewer interaction in tests by updating the element used for visibility checks.
  - Enabled opening the PDF viewer peek view in readonly and sharing modes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/affine-local/e2e/attachment-preview.spec.ts

@@ -316,7 +316,9 @@ test('should enable pointer event in pdf viewer', async ({ page }) => {
   await toolbar.getByLabel('Switch view').click();
   await toolbar.getByLabel('Embed view').click();
 
-  await attachment.locator('iframe').waitFor({ state: 'visible' });
+  await attachment
+    .locator('.affine-attachment-embed-container')
+    .waitFor({ state: 'visible' });
 
   const rect = await attachment.boundingBox();
   expect(rect).not.toBeNull();