fix(macx): crash in showFullScreen when QScreen is null + window jumping desktops

[raul-herreros]

Problem

When switching between Document mode and Board mode (e.g., double-clicking a page thumbnail), OpenBoard crashes with a SIGSEGV (EXC_BAD_ACCESS at address 0x8) on macOS with Qt 6.

Root Cause

In UBPlatformUtils::showFullScreen(), the original code calls:

QRect currentScreenRect = QGuiApplication::screenAt(pWidget->geometry().topLeft())->geometry();

QGuiApplication::screenAt() can return nullptr in Qt 6 (e.g., when the widget hasn't been fully mapped to a screen yet), and the code dereferences it without a null check, causing the crash.

Additionally, showMaximized() triggers macOS window manager behavior that can relocate the window to a different desktop/Space, causing the app to jump between screens unexpectedly when navigating between views.

Crash Stack Trace

Thread 0 Crashed:: CrBrowserMain
0   QtGui   QScreen::geometry() const + 0
1   OpenBoard   UBPlatformUtils::showFullScreen(QWidget*) + 168 (UBPlatformUtils_mac.mm:639)
2   OpenBoard   UBApplicationController::showBoard() + 136 (UBApplicationController.cpp:386)
3   OpenBoard   UBDocumentController::thumbnailPageDoubleClicked(...)

Fix

• Null-safety: Added a fallback chain for screen resolution: pWidget->screen() → QGuiApplication::screenAt() → QGuiApplication::primaryScreen(), with early return if all fail.
• Replaced showMaximized() with showNormal() + setGeometry(screen->geometry()) to avoid macOS window manager interference (no more window jumping to different desktops/Spaces).
• Added raise() + activateWindow() to ensure proper window focus.

Testing

• Tested on macOS Tahoe 26.5.1 (Apple Silicon) with Qt 6.11.1
• Verified switching between Document ↔ Board ↔ Document no longer crashes
• Window stays on the correct screen/desktop