Quick Send: align custom titlebar behavior and remove focus/z-order interference

[Copilot]

Quick Send diverged from the tray app’s window conventions: it used the system titlebar (including theme mismatch in dark mode) and force-kept foreground/topmost behavior that interfered with tray flyouts. This update aligns Quick Send with the existing custom titlebar model and stops it from reclaiming focus.

• Titlebar parity with Hub/Canvas windows

  • Enable ExtendsContentIntoTitleBar.
  • Introduce a custom 48px titlebar region and wire it via SetTitleBar(...).
  • Keep content layout sizing consistent by accounting for titlebar height in window size updates.

• Focus and z-order behavior

  • Remove forced topmost behavior (IsAlwaysOnTop) and topmost promotion logic.
  • Remove repeated foreground reacquisition loops.
  • Add deactivation dismissal so Quick Send closes when focus moves elsewhere (with a send-in-progress guard to avoid interrupting active submission).

• Maintainability touch-ups in the same path

  • Replace titlebar magic values with named constants (TitleBarHeight, reserved controls width, title icon constant) in QuickSendDialog to make intent explicit.

ExtendsContentIntoTitleBar = true;
SetTitleBar(titleBar);

private void OnActivated(object sender, WindowActivatedEventArgs args)
{
    if (args.WindowActivationState == WindowActivationState.Deactivated)
    {
        if (_isSending) return;
        Close();
        return;
    }

    RequestInputFocus();
}