docs(preview): proof-of-running WinUI 3 screenshots (dark + light)

Two screenshots captured from the live TeamsISO.App.WinUI .exe at
1280×780, one per theme. Both prove the redesign renders end-to-end
on Windows 11 with WindowsAppSDK 1.8 and no view-model wiring yet:

* docs/preview/winui3-mainwindow-light.png — App.Current.RequestedTheme
  set to Light via ThemeManager. Wild Dragon "W" mark renders as cyan
  (#0E7C82) on cyan-muted (#E6F8F9) tile per the light-mode accent
  split from DESIGN.md. All other rail icons render at FgSecondary
  (#4A4B50) for AA contrast.
* docs/preview/winui3-mainwindow-dark.png — same render, dark theme.
  Wild Dragon mark uses the airy #97EDF0 cyan on the deeper
  cyan-muted (#1B3537) tile. Rail icons + section text at FgPrimary
  (#F4F4F6).

ThemeManager default reverted to "System" (the screenshot for dark
mode was taken with the default temporarily set to "Dark", then
reverted before this commit). The light-mode screenshot is what runs
when the OS app-mode is light, which is what happened on this build
host tonight.

These are the artifacts to point at when stakeholders ask "what does
the redesign look like in practice?" — they are the WinUI 3 .exe, not
the HTML preview.

src/TeamsISO.App.WinUI/Services/ThemeManager.cs

@@ -29,6 +29,8 @@ public sealed class ThemeManager
     }
 
     private readonly UISettings _uiSettings;
+    // Default: System (follow OS app-mode). Override at runtime via Set();
+    // persistence to UIPreferences.Theme lands in the view-model commit.
     private string _preference = "System";
 
     public string Preference => _preference;