* chore: add en-GB language

* chore: remove en-GB contents

* chore: prevent visitors from breaking demo
- prevent disabling password login
- prevent updating `memos-demo` user
- prevent setting additional style
- prevent setting additional script
- add some error feedback to system settings UI

* Revert "chore: add en-GB language"

This reverts commit 2716377b04ca684abd03f76c1661000fd6994be8.