feat: add language picker to logged-in footer and editor settings

- Rewrites LanguagePicker to use availableLanguages from ol-footer meta
  instead of subdomainLang (which is always empty in single-domain setup)
- Passes availableLanguages through layout-react.pug → ol-footer meta so
  React footer picks it up
- Adds InterfaceLanguageSetting component to the editor settings modal
  ("Spelling and language" tab) for use when no footer is present
- Adds interface_language key to all five locale files (en/fr/de/es/it)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/app/src/Features/User/UserController.mjs

@@ -580,7 +580,11 @@ async function setLanguage(req, res) {
   if (userId) {
     await User.findByIdAndUpdate(userId, { languageCode: lngCode }).exec()
   }
-  const redir = req.get('Referer') || '/project'
+  const returnTo = req.query.return_to
+  const redir =
+    typeof returnTo === 'string' && returnTo.startsWith('/')
+      ? returnTo
+      : req.get('Referer') || '/project'
   res.redirect(302, redir)
 }