Offer a component host only the configuration it can use

`ElementCallConfiguration` was every URL parameter, which put the widget
plumbing (widgetId, parentUrl, isWidget), the user's identity, the
homeserver, the analytics and Sentry settings, and the standalone app's
shared room secret in front of a host that has no business setting any
of them — the component reads none of those. It is now the behavioural
configuration a widget's URL can carry plus the two properties a host
has a say in: the theme and the background. The background is now
actually applied to the component's root, which it was not before.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

component/index.tsx

@@ -38,6 +38,7 @@ import {
   type ReactNode,
   type Ref,
   useEffect,
+  useLayoutEffect,
   useMemo,
   useState,
 } from "react";
@@ -55,13 +56,16 @@ import { CallView } from "../src/room/CallView";
 import { ErrorPage } from "../src/FullScreenView";
 import { ClientProvider } from "../src/ClientContext";
 import { HostBridgeProvider } from "../src/HostBridge";
-import { RootElementProvider } from "../src/RootElementContext";
+import { RootElementProvider, useRootElement } from "../src/RootElementContext";
 import {
   configurationForIntent,
   componentProperties,
+  type UrlConfiguration,
   type UrlParams,
   UrlParamsProvider,
+  type UrlProperties,
   UserIntent,
+  useUrlParams,
 } from "../src/UrlParams";
 import { MediaDevicesContext } from "../src/MediaDevicesContext";
 import { MediaDevices } from "../src/state/MediaDevices";
@@ -100,8 +104,16 @@ export {
 /**
  * How Element Call should behave. Everything is optional; anything left out
  * takes the default that {@link ElementCallProps.intent} implies.
+ *
+ * This is the behaviour a widget can be configured with through its URL, plus
+ * the two facts about the call a host has a say in: the theme to start in and
+ * the background. The rest of what a widget's URL carries — who the user is,
+ * how to reach the homeserver, where to report analytics, the shared secret of
+ * a room that is encrypted with one — a component host supplies by other
+ * routes, or not at all.
  */
-export type ElementCallConfiguration = Partial<UrlParams>;
+export type ElementCallConfiguration = Partial<UrlConfiguration> &
+  Partial<Pick<UrlProperties, "theme" | "background">>;
 
 export interface ElementCallProps {
   /**
@@ -178,9 +190,14 @@ export async function initializeElementCall(
   });
 }
 
-/** Applies the theme to the container, before it is painted. */
+/** Applies the theme and background to the container, before it is painted. */
 const Decoration: FC<{ children: JSX.Element }> = ({ children }) => {
   useTheme();
+  const { background } = useUrlParams();
+  const rootElement = useRootElement();
+  useLayoutEffect(() => {
+    rootElement.setAttribute("data-background", background);
+  }, [rootElement, background]);
   return children;
 };