feat(i18n): add Simplified Chinese desktop localization

[Asterracy]

Summary

• Add bundled desktop i18n with English fallback and Simplified Chinese (zh-Hans).
• Add a persisted system / en / zh-Hans language preference with immediate switching.
• Localize the desktop UI across onboarding, agents, channels, communities, forum, home, huddle, messages, profile, projects, settings, sidebar, shared UI, and workflows.
• Keep the macOS native application menu synchronized with the selected UI language.
• Add catalog parity checks, a hardcoded-string ratchet, a frozen zh-Hans glossary, and i18n regression tests.
• Document the branch as an unofficial localization in LOCALIZATION.zh-Hans.md.

Relationship to existing localization work

This PR is intentionally related to and overlaps the existing i18n work:

• Add i18n/localization infrastructure (starting with Spanish support) #2929 — i18n/localization infrastructure
• Feature: i18n / language selector for desktop UI (start with zh-CN) #7478 — desktop i18n and a language selector
• Feature request: add Simplified Chinese localization for Buzz Desktop #7662 — Simplified Chinese localization for Buzz Desktop
• feat(desktop): add i18n infrastructure with a Language setting (#2929) #5087 — initial desktop i18n infrastructure
• feat(desktop): add i18n with system language support #5152 — system-language follow-up to the initial infrastructure

The existing PRs provide the relevant architectural context. This branch presents a complete zh-Hans desktop implementation for maintainer review. If maintainers prefer #5152 or another base, this PR can be split, rebased, or closed in favor of that direction.

Verification

All checks below were rerun from a clean clone based on the current upstream main:

• pnpm exec node src/i18n/catalogValidation.mjs — pass (catalog parity ok)
• pnpm typecheck — pass
• pnpm check — pass; 2 warnings and 5 infos, no errors
• pnpm test — 6572 passed, 0 failed, 0 skipped
• pnpm build — pass; protected feature artifact matrix passed
• just desktop-tauri-check — pass

The locale catalog contains 4082 leaf keys in both English and Simplified Chinese, with no missing or empty Chinese values.

Scope and attribution

• Desktop client only; no relay, mobile, or web localization is claimed here.
• User content, agent output, code identifiers, model names, and protocol fields remain untranslated where appropriate.
• This is an unofficial community localization and does not claim Block endorsement or Buzz trademark rights.
• Implemented and reviewed by Asterracy with assistance from OpenAI Codex. The commit author and DCO sign-off are Asterracy.

Review request

Please advise whether maintainers prefer this as one complete locale contribution or as a sequence of smaller infrastructure, catalog, and surface PRs.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 77729abfb692b25a0f4ec4a69add86af2e32c0dd...3343dee36cb81ae87878cc4bf78a2c95d97237f1.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review 3343dee36cb81ae87878cc4bf78a2c95d97237f1 to authorize a new review.
Any previous review applies only to its recorded range.