Port openfang PR #502 i18n infrastructure to LibreFang

[houko]

Summary

Port the i18n work from RightNow-AI/openfang PR #502 into LibreFang, adapting branding, paths, and current repo layout.

Upstream reference: RightNow-AI/openfang#502

Scope

• Add CLI i18n foundation (fluent-based language bundles, runtime bootstrap from config.language)
• Add initial en and zh-CN locale packs for CLI
• Add dashboard i18n loader and served locale JSON files
• Expose locale routes from librefang-api
• Add dashboard language switcher and localized labels for key nav/status surfaces
• Adapt copied strings from OpenFang to LibreFang and repo-specific paths such as ~/.librefang/config.toml

Notes

• LibreFang already has a language field in KernelConfig, so the port should use that instead of introducing a new config concept.
• The upstream PR appears to localize only part of the CLI output; remaining CLI/TUI strings can be follow-up work if needed.
• Keep the first port conservative: avoid broad unrelated refactors while transplanting the i18n foundation.

Acceptance Criteria

• librefang initializes i18n from config at startup
• Dashboard serves locale JSON and can switch between en and zh-CN
• Core nav/status/auth strings update when language changes
• Build passes for librefang-cli and librefang-api