Reintroduce admin UI i18n / English support (PR #424 was reverted)

（中文摘要：#423 / #424 曾为管理界面实现过完整的 i18n（简中 / 繁中 / 英文），合并后因为分支意外回退了其他功能而被 revert（9c4dc71），之后似乎一直没有重新合并。想问一下英文界面是否还在计划中？）

Hi, thanks for the project — I run it on a small home server as a relay for Claude Code and it's been working well.

I'm a non-Chinese-speaking user, so I currently navigate the admin UI through browser translation. It mostly works, but gets rough in places like form validation messages and confirmation dialogs.

While checking whether an English UI already existed, I noticed that full i18n support (zh-CN / zh-TW / en, based on vue-i18n) was actually implemented in #423 / #424 and merged last September, but then reverted in 9c4dc71 because the branch had unintentionally rolled back other features. As far as I can tell it was never re-submitted after that, and I couldn't find an open issue tracking it — hence this one.

Is bringing back i18n (or even just an English locale) still on the roadmap? Since the translation work in #424 looked mostly complete, perhaps it could be rebased onto the current main. I'd be glad to help test the English side if that's useful.