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 -->

packages/frontend/core/src/modules/feature-flag/constant.ts

@@ -214,7 +214,7 @@ export const AFFINE_FLAGS = {
     description:
       'com.affine.settings.workspace.experimental-features.enable-pdf-embed-preview.description',
     configurable: !isMobile,
-    defaultState: false,
+    defaultState: true,
   },
   enable_editor_rtl: {
     category: 'affine',