Use a dedicated i18next instance instead of the global singleton

Element Call configured the global i18next singleton. When Element Call
runs embedded in a host application rather than as its own page, that
singleton belongs to the host, so configuring it would clobber the
host's translations.

Create Element Call's own instance in utils/i18n.ts, configure it in the
initializer, and pass it to components via <I18nextProvider>. Drop
.use(initReactI18next) from the initializer: it registers the instance
as react-i18next's global default, which is the global we are trying to
avoid. Tests and stories keep using it, so that they do not need to wrap
every render in a provider.

Two modules imported `t` directly from "i18next" and so were bound to
the global instance: utils/errors.ts now calls i18n.t() on the instance
(reached at call time, since i18next only assigns `t` during init), and
QrCode uses useTranslation() like every other component.

No functional change.

src/QrCode.tsx

@@ -8,7 +8,7 @@ Please see LICENSE in the repository root for full details.
 import { type FC, useEffect, useState } from "react";
 import { toDataURL } from "qrcode";
 import classNames from "classnames";
-import { t } from "i18next";
+import { useTranslation } from "react-i18next";
 
 import styles from "./QrCode.module.css";
 
@@ -18,6 +18,7 @@ interface Props {
 }
 
 export const QrCode: FC<Props> = ({ data, className }) => {
+  const { t } = useTranslation();
   const [url, setUrl] = useState<string | null>(null);
 
   useEffect(() => {