UX - Add solid opaque background to untextured canvas items

[DevGeniusCode]

Title:
UX: Add solid opaque background to untextured canvas items

Description:
Currently, UI controls on the canvas are fully transparent if they don't have a specific texture loaded. Since all windows and sub-panes are visible on the canvas simultaneously, this causes severe visual clutter (especially when working with Tab sub-panes stacked on top of each other). We need to give these items a solid background color.

Tasks:

• Locate the custom QGraphicsItem class responsible for drawing UI controls (e.g., WndGraphicsItem).
• Modify its paint method.
• Check if a texture/image is available. If NO texture is present, fill the bounding rectangle with a solid, dark, opaque color (e.g., QColor(45, 45, 48, 255)) before drawing the outline and text.

Acceptance Criteria:

• Items without textures render with a dark background instead of being transparent.
• When dragging one window over another, the top window successfully hides the content of the window beneath it.
• Text and borders are still drawn correctly on top of the new background.