follow-up: Migrate to `useIsMobile` / `useIsFullscreen`

[Antreesy]

nextcloud/vue v8.1.0 bring along new composables (useIsMobile, useIsFullscreen), which we should use instead of old mixins.

[Antreesy]

None of js, mjs, cjs doesn't work here.
Am I missing something, @ShGKme ?