Summary

This pull request:

• Applies the zoom level setting to all windows when zooming in/out.
• On creation, initializes secondary windows with the same zoom level as the main window.
  When the zoom level changes, this pull request

Fixes #11444.

Testing plan

This pull request has been tested manually by:

1. Starting Joplin.
2. Opening a note in a new window.
3. Pressing ctrl-+ in the secondary window.
4. Verifying that both the secondary and main windows zoom in.
5. Opening a different note in a new window.
6. Pressing ctrl-0.
7. Verifying that all windows zoom out.

This has been tested successfully on Fedora (Linux) 41.