Test and fix multi-monitor and high-DPI capture behavior

[rancy777]

Goal

Find and fix capture-coordinate and UI problems across multiple monitors and non-100% DPI scaling.

Suggested matrix

• One and two monitors
• Mixed monitor resolutions
• 100%, 125%, 150%, and mixed scaling
• Monitors positioned left/right and above/below the primary monitor
• Screenshot selection, recording region, HUD placement, and output dimensions

Include before/after evidence and the tested setup in the pull request.