Lay the party logo over the background

Centred, spanning 60% of the screen width, on the poster's layer so the
video frame and the stills still pass in front of it. The file lives in
assets/ with the posters, so it has to be kept out of the background
rotation — it is the one image there that is never a background.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9v2Xoa6eoeGYpK6gp95k9

public/player/backdrop.js

@@ -17,11 +17,24 @@ import {
   BACKGROUND_MAX_ZOOM,
   BACKGROUND_FOCUS_SPREAD,
   BACKGROUND_MAX_OVERSAMPLE,
+  LOGO_SRC,
+  LOGO_WIDTH,
 } from '/config.js';
 import { createPicker } from './motion.js';
 
 const KEYS = Object.keys(BACKGROUNDS);
 
+/**
+ * Point the logo overlay at the configured file and size it against the
+ * screen. The element is in the page from the start — only the two values that
+ * belong to the config are set here.
+ */
+export function showLogo(element) {
+  if (!element) return;
+  element.src = LOGO_SRC;
+  element.style.width = `${LOGO_WIDTH * 100}vw`;
+}
+
 /**
  * A zoom level and the point of the image the crop closes in on, in percent.
  * The focal point is pulled toward the middle by BACKGROUND_FOCUS_SPREAD, so a