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/RootElementContext.ts

@@ -16,17 +16,18 @@ import { createContext, use } from "react";
  * that when embedded in a host application it becomes the container the host
  * mounted it into, so that Element Call does not reach outside its own subtree.
  *
- * That intent is not yet achievable: several selectors still name `body`
- * directly — `body[data-background="gradient"]` and `body[data-platform=…]` in
- * `index.css`, and `body[data-platform="ios"]` in `AppBar.module.css` and
- * `Modal.module.css` — and `Initializer.initBeforeReact` writes
- * `data-platform` straight onto the body. So anything other than the body will
- * be decorated correctly and styled incorrectly, silently. Until those are
- * scoped, treat this as preparation rather than a working seam.
+ * The stylesheets find this element by its `data-element-call-root` attribute,
+ * which {@link useTheme} sets along with the platform and theme, so they no
+ * longer depend on it being the body.
+ *
+ * What remains body-specific is the standalone page's own furniture: the
+ * `body` rule in `index.css` still sets the page background and margin, and
+ * `index.html` starts the body hidden with `no-theme` until the theme lands.
+ * Neither applies when a host mounts Element Call into a container of its own.
  */
 // No provider is exported yet: nothing supplies a root element, so every
-// consumer falls back to the document body. M1 adds one along with the
-// component that mounts Element Call into a container.
+// consumer falls back to the document body. One arrives with the entry point
+// that mounts Element Call into a container.
 const RootElementContext = createContext<HTMLElement | null>(null);
 
 /**