Find Element Call's root by attribute, not by being the body

Six selectors named `body` directly — the gradient backdrop and the
platform font overrides in index.css, and the iOS adjustments in
AppBar.module.css and Modal.module.css — so they only applied when Element
Call owned the page. A host mounting it into a container would have got an
interface decorated correctly and styled incorrectly, with nothing to show
that anything was wrong.

Mark the root element with data-element-call-root and match on that
instead. Scoping this way keeps the selectors more specific than they were,
rather than less: widening them to a bare [data-platform=…] would have
dropped specificity from (0,1,1) to (0,1,0) and changed which rules win.

The platform attribute moves with them, from the initializer's write onto
document.body to a layout effect on the root, alongside the theme — so it
still lands before anything is painted.

No visual change while Element Call owns the page: the root is the body,
which now carries the attribute, so every rewritten selector matches the
element it always did.

src/initializer.tsx

@@ -30,7 +30,6 @@ import {
 import { getUrlParams } from "./UrlParams";
 import { Config } from "./config/Config";
 import { seedSettingsFromConfig } from "./settings/settings";
-import { platform } from "./Platform";
 import { isFailure } from "./utils/fetch";
 import { initializeWidget, type WidgetHelpers } from "./widget";
 import { enableExtendedLivekitLogs } from "./settings/settings.ts";
@@ -229,9 +228,6 @@ export class Initializer {
       );
     }
 
-    // Add the platform to the DOM, so CSS can query it
-    document.body.setAttribute("data-platform", platform);
-
     // livekit logging configuration
     setLKLogExtension((level, msg, context) => {
       // we pass a synthetic logger name of "livekit" to the rageshake to make it easier to read