feat: add multi-language localization

[htazq]

Summary

• Add a lightweight i18n layer for auto, en-US, zh-CN, de-DE, ja-JP, and ru-RU with system-language detection and English fallback.
• Add a Settings language selector persisted in sshClientSettings.language and refresh UI immediately on change.
• Localize primary UI flows: menus, connection management, connection dialog, settings, monitor/log/file actions, welcome/status/restore overlays, terminal menus, and key toast messages.
• Rebuild the native macOS menu from the current locale via set_app_locale; Windows/Linux keep React menu rendering.
• Display-localize default folders (All Connections, Personal, Work) without changing stored values.
• Keep test execution reliable by making SSH integration Vitest opt-in and validating SSH public key files before network connect.

Fixes #9.
Addresses #21.

Tests

• pnpm test — 412 passed, 7 skipped SSH integration tests by default
• pnpm lint — 0 errors, existing warnings remain
• pnpm build — passed
• cd src-tauri && cargo test — 76 passed, 5 ignored