Stop asking whether Element Call is a widget

The remaining reads of the widget global were all asking one of two
different questions, so they get two different answers.

The app shell — the auth hooks, automatic guest registration, the group
call loader's diagnostic and the initial mute state — wants to know
whether Element Call was launched as a widget. That is a property of the
URL it was launched with, so expose the isWidget that computeUrlParams
already computed internally, documented as being for shell use only.

The call interface — whether to offer the profile settings tab — wants to
know something about its host, so it asks the bridge. A host that can
dismiss Element Call owns the user's account, so their profile is not ours
to edit; this reuses the close capability as a proxy, with a TODO
alongside the others.

ClientContext also takes supportsReactions from the bridge rather than
checking four widget capabilities itself, which removes widgetApi from
InitResult — a field that was always null outside widget mode.

Note this changes behaviour for a malformed widget URL: one carrying a
widget ID and parent URL but missing the room, user, device or base URL
would previously have fallen back to registering a guest user, and will
now not.

src/auth/useInteractiveRegistration.ts

@@ -17,7 +17,7 @@ import { logger } from "matrix-js-sdk/lib/logger";
 import { initClient } from "../utils/matrix";
 import { type Session } from "../ClientContext";
 import { Config } from "../config/Config";
-import { widget } from "../widget";
+import { useUrlParams } from "../UrlParams";
 
 export const useInteractiveRegistration = (
   oldClient?: MatrixClient,
@@ -32,6 +32,7 @@ export const useInteractiveRegistration = (
     passwordlessUser: boolean,
   ) => Promise<[MatrixClient, Session]>;
 } => {
+  const { isWidget } = useUrlParams();
   const [privacyPolicyUrl, setPrivacyPolicyUrl] = useState<string | undefined>(
     undefined,
   );
@@ -47,7 +48,7 @@ export const useInteractiveRegistration = (
   }
 
   useEffect(() => {
-    if (widget) return;
+    if (isWidget) return;
     // An empty registerRequest is used to get the privacy policy and recaptcha key.
     authClient.current!.registerRequest({}).catch((error) => {
       setPrivacyPolicyUrl(
@@ -55,7 +56,7 @@ export const useInteractiveRegistration = (
       );
       setRecaptchaKey(error.data?.params["m.login.recaptcha"]?.public_key);
     });
-  }, []);
+  }, [isWidget]);
 
   const register = useCallback(
     async (