Build Element Call as a component a host can import

Adds component/index.tsx as a fourth build target: <ElementCall client
roomId /> and an initializeElementCall to await once beforehand. It gives
Element Call everything it would otherwise take from the page it is on —
the parameters, the host bridge, media devices, translations, a container
to confine itself to — and hands it the host's client rather than finding
one of its own.

React, the Matrix SDK and LiveKit stay external, since the host has them
and a second copy of any would not merely be wasteful: React would hold
two sets of hooks and the client would run two sync loops. Every subpath
has to be listed by name, because the pattern and callback forms of
rollupOptions.external are silently ignored here — a lesson worth the
comment that records it.

Element Call's own navigation runs in a MemoryRouter, so being embedded
cannot disturb the host's URL. ClientContext and GroupCallView both
navigate, so some router has to be present.

The bundle is not yet a reasonable size: library mode base64-inlines
assets referenced through import.meta.url, so MediaPipe's vision runtime
lands in it whole. Left for its own change, since the fix — loading the
background blur transformer lazily — is worth doing for the standalone app
too.

src/RootElementContext.ts

@@ -25,11 +25,14 @@ import { createContext, use } from "react";
  * `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. One arrives with the entry point
-// that mounts Element Call into a container.
 const RootElementContext = createContext<HTMLElement | null>(null);
 
+/**
+ * Supplies the element Element Call should confine itself to. The standalone
+ * and widget builds need no provider, since for them that element is the body.
+ */
+export const RootElementProvider = RootElementContext.Provider;
+
 /**
  * The element Element Call should decorate and portal into.
  *