design: preview pane to tokens (surface 4), and one palette list instead of two

110 hardcoded colours gone from the preview — the biggest surface in the app. Nicknames:
- the webcam selection ring is the interactive accent (was the retired brand red)
- the mute chips are `attention` (a muted mic is a caution, not a live state) with a white play
  glyph; the live badge is `primary`
- YPP progress bars/track, TRAX level zones, the error line, the dim strips and the poster veil
  all resolve from tokens (`positive`, `secondary-container`, `error-container`, `scrim-dark`,
  `scrim-dark-strong`)

Plus the duplication this sweep exposed: the webcam's right-click "Border Color" submenu carried
**seven hand-written colours**, a second source of truth beside the properties drop-down that
TASK 63 had already unified onto `Models/BorderColorPalette.cs` — and the two lists disagreed
(7 vs 14). The submenu is now built from the palette when it opens, so every offered colour is
the same colour everywhere, with the palette's name beside it. The click handler is unchanged;
generated rows inherit the menu's SceneElement DataContext.

Guard list extended: Controls/PreviewPane.xaml.

ytLive.Tests/DesignTokensTests.cs

@@ -75,7 +75,8 @@ public class DesignTokensTests
         // guarded on its own instead of waiting for a global grep at the end. Pure white and black
         // are allowed (gloss highlights, knockout masks) — everything else must be a token.
         string[] converted = { "Themes/Controls.xaml", "MainWindow.xaml", "Controls/TopBar.xaml", "Controls/SceneThumbnailStrip.xaml",
-                                 "Controls/LeftPanel.xaml" };
+                                 "Controls/LeftPanel.xaml",
+                                 "Controls/PreviewPane.xaml" };
 
         foreach (var file in converted)
         {