fix: Deprecation warning, set `ssr: false`

mode option is deprecated. Please use ssr: true for universal mode or ssr: false for spa mode and remove mode from nuxt.config

Dependency blueimp-load-image calls document and is therefore not usable in server-side-rendering. SSR is a must to improve SEO. Also going full static would be a great improvement for this app: See: https://nuxtjs.org/blog/going-full-static/#new-config-option-target