[studio][Feature] Add a localized not-found page for unknown Studio routes

## Problem Unknown Studio URLs are silently redirected to the home page. Users lose the original context and cannot tell whether a bookmarked route is invalid, removed, or mistyped. ## Proposal Render an explicit localized not-found view for unmatched routes. The view should explain what happened in both supported languages and provide a clear action back to the Studio home page. ## Acceptance criteria - Unknown routes render a dedicated 404/not-found view instead of silently redirecting. - English and Chinese copy are available through the existing language preference. - Users can navigate back to the home page with one action. - Route behavior is covered by frontend tests. Target branch: rocketmq-studio.