Make settings window independent and taskbar-aware

Convert the settings window into an independent top-level window with its own taskbar icon and open-or-focus semantics. Removed Owner/anchor/toggle semantics from SettingsWindowService and added ScreenReferenceWindow for centering; settings windows now ShowInTaskbar = true and are truly destroyed on close. Added SettingsWindowPlacementHelper and tests for placement/centering. Main window now respects an AppSettingsSnapshot.ShowInTaskbar flag (new setting exposed in GeneralSettings UI) and slide/visibility animations and "back to Windows" behavior no longer affect the independent settings window. Updated various callers to use OpenIndependentSettingsModule, adjusted window transitions/X offsets, and added/updated spec files documenting the feature and animation boundary.

LanMountainDesktop.Tests/SettingsWindowPlacementHelperTests.cs

@@ -0,0 +1,50 @@
+using Avalonia;
+using LanMountainDesktop.Services.Settings;
+using Xunit;
+
+namespace LanMountainDesktop.Tests;
+
+public sealed class SettingsWindowPlacementHelperTests
+{
+    [Fact]
+    public void ResolveWorkingArea_PrefersReferenceScreen()
+    {
+        var referenceArea = new PixelRect(1920, 0, 2560, 1440);
+        var primaryArea = new PixelRect(0, 0, 1920, 1080);
+
+        var result = SettingsWindowPlacementHelper.ResolveWorkingArea(
+            referenceArea,
+            primaryArea,
+            fallbackWindowWidth: 1120,
+            fallbackWindowHeight: 760);
+
+        Assert.Equal(referenceArea, result);
+    }
+
+    [Fact]
+    public void ResolveWorkingArea_FallsBackToPrimaryScreenWhenReferenceIsMissing()
+    {
+        var primaryArea = new PixelRect(0, 0, 1920, 1080);
+
+        var result = SettingsWindowPlacementHelper.ResolveWorkingArea(
+            referenceWorkingArea: null,
+            primaryWorkingArea: primaryArea,
+            fallbackWindowWidth: 1120,
+            fallbackWindowHeight: 760);
+
+        Assert.Equal(primaryArea, result);
+    }
+
+    [Fact]
+    public void CalculateCenteredPosition_ReturnsCenteredPointInsideWorkingArea()
+    {
+        var workingArea = new PixelRect(1920, 40, 2560, 1400);
+
+        var result = SettingsWindowPlacementHelper.CalculateCenteredPosition(
+            workingArea,
+            windowWidth: 1120,
+            windowHeight: 760);
+
+        Assert.Equal(new PixelPoint(2640, 360), result);
+    }
+}