Draw the game view letterbox in the editor background colour

The bars around the camera frame in the Game tab were pure black.
They now take Theme.colors.background, so the tab blends in with the
rest of the editor instead of framing the game in a black box.

- GameViewport drops LETTERBOX_COLOR and uses the theme colour for
  both the letterbox and the model frame.
- e2e 3f checked "the crate is drawn, not letterbox" against
  #000000. With the new colour that would pass whatever is drawn,
  so it now compares against #1b1d22.

Tests:
- ./check.sh: 178 files, 2575 tests, CHECK EXIT 0
- GM_STEPS=1-3,46: ALL PASSED, 94 checks (3f reads #e0553f)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/e2e/03-play.mjs

@@ -254,7 +254,8 @@ export async function play(t) {
   );
   check(
     'and the crate is really drawn there, not left as letterbox',
-    previewPixel !== '#000000',
+    // The letterbox is Theme.colors.background.
+    previewPixel !== '#1b1d22',
     previewPixel,
   );
   await shot('10c-game-preview');