Hide viewport overlay when other windows are docked atop of the viewport

[tarcila]

Changed the overlay from a separate top-level window to a child window within the viewport. This ensures the overlay is properly occluded when other windows are docked on top of the viewport. Otherwise, other windows such the tranfer function editor would render behind the overlay.

[Copilot]

Pull request overview

This PR changes the viewport overlay rendering approach to fix z-ordering issues with docked windows. The overlay is converted from a standalone top-level window to a child window within the viewport, ensuring it's properly hidden when other windows (like the transfer function editor) are docked on top.

Changes:

• Removed separate overlay window implementation and replaced it with a child window
• Relocated overlay rendering and menu bar positioning in the build UI flow
• Added window flags to prevent scrolling in the viewport

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tsd/src/tsd/ui/imgui/windows/Viewport.h	Removed the m_overlayWindowName member variable no longer needed for child window implementation
tsd/src/tsd/ui/imgui/windows/Viewport.cpp	Refactored overlay from top-level window to child window, updated positioning logic, and adjusted rendering order

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[jeffamstutz]

Nice -- LGTM, thanks!