EAF pdf viewer loses focus

**Describe the bug**
The installation documentation does not mention that app-frontmost needs to be installed, and it is not installed on my Mac — I also don’t know how to install it. When using EAF to view PDFs, no error is reported, but after switching windows while viewing a PDF, it is easy for the viewer to lose focus.

**To Reproduce**
Viewing PDFs in EAF，after switching window and back to emacs, the PDF viewer loses focus.

**Expected behavior**
switch back to emacs, keep focus on pdf

**Versions (please complete the following info):**
- Distro and DE/WM:
- Versions of Dependencies:
- M-x emacs-version: GNU Emacs 31.0.50 (build 5, aarch64-apple-darwin24.1.0, NS appkit-2575.20 Version 15.1.1 (Build 24B2091)) of 2025-04-11

**Error logs**
2025-08-28 21:02:27.788 Python[38744:2667269] +[IMKClient subclass]: chose IMKClient_Modern
2025-08-28 21:02:27.788 Python[38744:2667269] +[IMKInputSession subclass]: chose IMKInputSession_Modern

**Screenshots**

**Additional context**

Start with the installation documentation and reproduce PDF viewing in EAF on macOS, including the reported missing app-frontmost dependency. Then trace the PDF viewer's focus behavior after switching windows. Done means the dependency guidance and the focus behavior are addressed, with the result verified on the stated Emacs and macOS versions.

Written by the indexing model from the issue text.