wip: Windows desktop app scaffold (WinUI 3/.NET 8) + NSIS installer + docs

Owner decision pending (STATE.md DC-100 tick): adopt/ship/park.
Preserved from fragile git stash to named branch 2026-08-23.
NOTE: requires a Windows build machine (WinUI XAML compiler + MSIX do not
cross-build on Linux) — see WINDOWS_APP_BUILD.md in this tree.
Secret-scanned clean 2026-08-23 (no keys/tokens/PEM in tree).

desktop/App.xaml

@@ -0,0 +1,17 @@
+<Application
+    x:Class="DashCaddy.Desktop.App"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:DashCaddy.Desktop">
+    
+    <Application.Resources>
+        <ResourceDictionary>
+            <ResourceDictionary.MergedDictionaries>
+                <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
+                <!-- Custom styles -->
+                <ResourceDictionary Source="Styles/Colors.xaml" />
+                <ResourceDictionary Source="Styles/Controls.xaml" />
+            </ResourceDictionary.MergedDictionaries>
+        </ResourceDictionary>
+    </Application.Resources>
+</Application>