feat: add full Spanish translation (#57)

* feat(i18n): add spanish translation support

* refactor(i18n): refine spanish copy for es-es

* refactor(i18n): translate addon titles to spanish

client/src/pages/SettingsPage.tsx

@@ -269,6 +269,7 @@ export default function SettingsPage(): React.ReactElement {
                 {[
                   { value: 'de', label: 'Deutsch' },
                   { value: 'en', label: 'English' },
+                  { value: 'es', label: 'Español' },
                 ].map(opt => (
                   <button
                     key={opt.value}