feat(dashboard): add per-account proxy field to single-add form

Add optional proxy input to dashboard single-account add form. Mirrors batch-import proxy binding — user can specify http://proxy:8080 or socks5://user:pass@host:port per account. Backend validates format via parseProxyUrl(), respects ALLOW_PRIVATE_PROXY_HOSTS config (validateHostFormat vs assertPublicUrlHost), calls setAccountProxy() + ensureLsForAccount(). UI clears proxy field on success. i18n for EN/zh-CN.