Snap widget to taskbar regions and resize bars to fit

[Helveg]

I added a bit of layout logic so that the widget can be easily placed into open places on the taskbar, and also resizes and relayouts on changes.

Summary

• Dragging the widget now shows translucent highlights over open taskbar regions; the one under the cursor lights up, and dropping snaps the widget into it.
  Releasing outside any valid region reverts to the pre-drag position.
• The widget resizes to fill the dropped region by stretching its progress-bar segments (clamped between 5 and 12 design-pixels). Regions on the left half of the
  taskbar left-align the widget; regions on the right half right-align.
• Open regions are computed from HWND enumeration plus a UIA scan of XAML hosts (DesktopWindowContentBridge) so Win11 pinned/running apps are accounted for. The
  UIA cache refreshes off the UI thread on a 500 ms tick and after every drag, and the widget auto-resizes if its current region shrinks below the minimum fit (jumping
  to the nearest valid region).
• New --debug-render flag overlays red-bordered boxes with class+size labels around every detected occupant, for diagnosing detection.