refactor(ui): route logging through IPluginLogProxy

F12.2 step 5a — UI cluster (~40 sites in 6 files):
ChatLogWindow, DbViewer, Popout, SettingsTabs/{DataManagement,
FontsAndColours, ThemeAndLayout}. Plugin.Log.X(...) → Plugin.LogProxy.X(...).
No behaviour change; the proxy delegates 1:1 to the original IPluginLog.

HellionChat/Ui/SettingsTabs/ThemeAndLayout.cs

@@ -90,7 +90,7 @@ internal sealed class ThemeAndLayout : ISettingsTab
                 var path = Path.Combine(dir, fileName);
                 var json = ThemeJsonWriter.Serialize(active);
                 File.WriteAllText(path, json);
-                Plugin.Log.Information($"Exported active theme '{active.Slug}' to {path}");
+                Plugin.LogProxy.Information($"Exported active theme '{active.Slug}' to {path}");
             }
         }
     }