add translation keys (#246)

* Fix i18n tsc issue

"t" can return null by default.
This was already disabled in the config, but it had no effect on tsc.

* Fix translation in "DefaultNavBar"

key was missing and since this is a constant the translation wouldn't update in case the language got changed after first load.

* Enable typing for i18n translation keys

* Add translation keys

src/screens/Browse.tsx

@@ -30,8 +30,8 @@ export default function Browse() {
                 scrollButtons
                 allowScrollButtonsMobile
             >
-                <Tab sx={{ textTransform: 'none' }} label={t('screens.Browse.sources')} />
-                <Tab sx={{ textTransform: 'none' }} label={t('screens.Browse.extensions')} />
+                <Tab sx={{ textTransform: 'none' }} label={t('source.title')} />
+                <Tab sx={{ textTransform: 'none' }} label={t('extension.title')} />
             </Tabs>
             <TabPanel index={0} currentIndex={tabNum}>
                 <Sources />