[222_45] compact toolbar and fix native menu bar empty strip (#2877)

[AnvarAtayev]

Closes Issue #2877

Summary

Two related UI improvements for the liii/liii-night themes on macOS:

1. Compact toolbars - reduce toolbar row height from 36 px to 28 px and tighten button padding, giving a denser, more native-feeling toolbar layout.
2. Fix empty white strip with native menu bar - when use_native_menubar is enabled, the menuToolBar container remained visible at full height even though its content moved to the system menu bar. The toolbar is now hidden in this case.

Also removes an incorrect setFixedHeight(108 * devicePixelRatio) on the QMenuBar widget, which was setting a physical-pixel value on a logical-pixel widget (resulting in a 216 px tall menu bar on Retina displays).

Files Changed

• TeXmacs/misc/themes/liii.css
• TeXmacs/misc/themes/liii-night.css
• src/Plugins/Qt/qt_tm_widget.cpp

How to Test

• Build and launch with the default (liii) theme; verify toolbar rows are more compact
• Enable use_native_menubar in Preferences -> User Interface; verify no empty strip remains below the title bar
• Disable use_native_menubar; verify the in-window menu bar returns correctly

OLD - use_nativebar=True

OLD - use_nativebar=False

NEW