Fix menubar in fullscreen

[YukiNagat0]

Fixes: #3647

As alternative to: #3682

Demo:

1.mp4

[YukiNagat0]

Notes:
The menubar hiding behavior has been implemented with two key considerations:

1. To prevent flickering, the menubar only hides once when entering the deck browser, maintaining the current master branch behavior.
2. A fullscreen check was added to _reviewState():
   

   anki/qt/aqt/main.py

   Lines 796 to 807 in c3a21f0

   	def _reviewState(self, oldState: MainWindowState) -> None:
   	self.reviewer.show()
   	if self.fullscreen:
   	self.hide_menubar()
   	if self.pm.hide_top_bar():
   	self.toolbarWeb.hide_timer.setInterval(500)
   	self.toolbarWeb.hide_timer.start()
   	else:
   	self.toolbarWeb.flatten()

   
   to prevent a potential menubar softlock on entering reviewer because of 1. . Without this check, the menubar could become stuck until manually hovered over it in reviewer.
   The quirk with this approach is that is causes the menubar to hide instantly when entering review state, bypassing the usual 500ms delay.
   An alternative approach would be to modify the if self.pm.hide_top_bar() check to: if self.fullscreen or self.pm.hide_top_bar(), it will trigger timer which will call hide_menubar in the hide_if_allowed method.
   While this would maintain consistent timing with the default flow (hiding via timer trigger with 500ms delay), it would affect the toolbarWeb.flatten() behavior.
3. Except small qurik in the 2., this PR preserves original logic and unifies menubar behavior, regardless of the Hide top bar setting.

[YukiNagat0]

For more context, please read the original issue #3647 and these comments: #3682 (comment) and #3682 (comment)

[KartikSharma0]

@YukiNagat0 Just looked over your PR, I now understand what the intended functionality of the original code was. I appreciate your effort in trying to explain it to me though!

[dae]

Thank you both! I find number 1 a bit confusing, but as you said, that seems to be existing behaviour. If @abdnh is happy with this, good to go.

[abdnh]

Looking good to me.