Qt: Add View menu toggle for main window fullscreen

[Cho1409]

Description of Changes

Adds a checkable "Fullscreen Main Window" entry to the View menu. Toggling it shows/restores the main window in fullscreen and persists the existing UI/MainWindowFullscreen setting. The menu item is also synchronized with the saved setting on startup.

Rationale behind Changes

The MainWindowFullscreen setting was added in #13249, but until now it could only be enabled by manually editing PCSX2.ini. This exposes it through the GUI so it can be turned on/off without hand-editing the config file. Closes #14405.

Suggested Testing Steps

1. Open the main window (game list).
2. Use View → Fullscreen Main Window and confirm the window enters fullscreen; toggle again to restore.
3. Enable it, close PCSX2, and reopen — the main window should start in fullscreen and the menu item should be checked.
4. Disable it and confirm MainWindowFullscreen = false is written and the window starts normally on next launch.

Did you use AI to help find, test, or implement this issue or feature?

Yes — as an assistant for locating the relevant code, researching how the existing MainWindowFullscreen setting is read/written, and validating the build. I understand and can fully explain the change; the menu action follows the same pattern as the existing View toggles (toolbar/status bar).

[chaoticgd]

Looks like this is the same as #14566. That one did forget about the signal blocker though.

[Cho1409]

@chaoticgd Oh you're right, I didn't see that, my bad. Feel free to take his solution btw.

[chaoticgd]

I think we'll go with the first one, seems fair. Looks like the signal blocker isn't strictly required in this case since QAction::triggered only fires on click (although the other checkboxes do use it so I can see why it was added).