fix(i18n): localize the studio user management page

[yyqdbngt]

Summary

Localize the Studio user management page (studio/UserManagement.tsx), the last fully hard-coded page on the web side:

• Wire useLang() into the page
• Replace ~45 hard-coded strings with t() calls and new studioUser.* zh/en keys: 10 feedback toasts (load/create/enable/disable/password/export), table columns (用户名/用户 ID/权限/状态/创建时间/操作), role and status tags, page header + card title, admin-denied banner, search/filter placeholders and aria-labels, role/status filter options, create-user form labels and validation messages, password-change/reset modal labels and titles
• The pagination showTotal stays untouched here — the shared common.totalUsers key is introduced by the dedicated pagination-totals PR (fix(i18n): localize pagination totals across instance and studio pages #2973) to avoid duplicate keys

Why

The page rendered entirely in Chinese in the English UI — every toast, column header, filter and form label was hard-coded.

Testing

• vitest run src/pages/studio/__tests__/UserManagement.test.tsx → 4/4 passed (zh default render unchanged)
• tsc --noEmit → clean
• eslint on changed files → 0 problems

[RockteMQ-AI]

Summary

Clean i18n localization — replaces hard-coded Chinese strings with t() calls and adds corresponding zh/en translation keys. Changes are small, focused, and follow the established i18n pattern in the codebase.

Correctness ✅ — All string replacements map correctly to the new translation keys.
Compatibility ✅ — No behavioral changes, only UI string localization.
Completeness ✅ — Both zh and en translations provided for all new keys.

LGTM.

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