fix(ui): embed popups to fix ComboBox dropdown placement on X11 HiDPI

[obazna]

Problem

On X11 with HiDPI + GNOME fractional scaling (Xft.dpi 192 → 2× scale, experimental x11-randr-fractional-scaling + scale-monitor-framebuffer), ComboBox dropdowns open in the wrong place. The FlightData → Actions → Set Mode flight-mode list flips up over the HUD, and intermittently lands jammed in the top-left screen corner, clipped, detached from its combo.

Cause

Popups default to separate X11 override-redirect windows positioned by Avalonia's ManagedPopupPositioner. Under fractional scaling that positioner miscomputes placement (wrong flip / stale anchor translation), so the dropdown detaches from its combo. The controls are plain <ComboBox> (e.g. GCSViews/FlightDataView.axaml) with no custom placement — this is a platform-positioner issue, not a XAML one.

Fix

Set X11PlatformOptions.OverlayPopups = true in Program.cs BuildAvaloniaApp(), so popups render inside the top-level window's overlay layer and bypass the X11 positioner entirely. One-line, app-wide (covers combobox dropdowns, menus, flyouts, tooltips).

AppBuilder.Configure<App>()
    .UsePlatformDetect()
    .With(new X11PlatformOptions { OverlayPopups = true })
    .WithInterFont()
    .LogToTrace();

Verification

• Build: 0 warnings / 0 errors (dotnet build -c Debug).
• Launched on the affected 5120×2880 / 2× display; opened Actions dropdowns — they now render in-window, anchored directly below the combo, correctly aligned. Confirmed via X11 window-tree diff that no separate popup window is created while the dropdown is open (before: a 390×1008 override-redirect window appeared, mispositioned).

Trade-off

Overlay popups are clipped to the window bounds. The main window is effectively full-screen, so this is a non-issue here; small dropdowns stay glued to their combo and scroll when tall.