Fix custom title-bar dragging

[HC4074]

What changed

• Made the complete custom title bar a hit-testable drag surface.
• Restricted drag handling to the left mouse button and normal window state.
• Preserved double-click maximize and restore behavior.

Root cause

The title-bar border had no background, so its empty area did not participate in WPF hit testing. Most of the visible bar therefore never delivered the mouse event that calls DragMove().

Validation

• Published the self-contained Release build successfully.
• Launched the packaged executable and confirmed the main window initializes.