Localize MCP argument step copy

[w287346141]

Summary:

• Localize the filesystem MCP argument step summary and sandbox note.
• Keep the English strings unchanged through the i18n layer.
• Add wizard test coverage for the zh-CN argument-step copy.

Verification:

• npm.cmd run test -- wizard.test.tsx
• npm.cmd run typecheck
• npm.cmd run verify

[w287346141]

Superseded by #21, which batches the setup wizard localization work and resolves the shared Wizard.tsx / tests/wizard.test.tsx conflicts ahead of review.