feat: complete Simplified Chinese localization

[maopai]

Summary

• complete the Simplified Chinese React dashboard coverage across navigation, diagnostics, reports, calls, threads, limits, cache/context, tables, and visualization metadata
• add a scoped translation layer for dashboard strings while marking user-provided values so thread names, project names, paths, and other source data remain unchanged
• localize visualization specs before chart and synchronized-table rendering, including accessibility descriptions
• localize CLI help plus dashboard lifecycle messages when --lang zh-Hans is selected
• normalize Chinese terminology to use token consistently and rebuild the bundled dashboard assets
• document the expanded Simplified Chinese behavior in README and CHANGELOG

Verification

• TZ=UTC npm run test -- --run (437 passed)
• npm run typecheck
• npm run lint
• npm run build
• .venv/bin/python -m pytest -q tests/core/test_i18n.py tests/cli/test_cli_dashboard.py tests/cli/test_cli_help_i18n.py tests/server/test_server_dashboard_shell.py tests/server/test_server_api.py (144 passed)
• .venv/bin/ruff check ... for all changed Python and Python test files
• git diff --check
• manually sampled codex-usage-tracker --lang zh-Hans --help and the live dashboard

Review note

This is opened as a draft because the React localization bridge is broader than the existing JSON catalog. Early feedback on that architecture is welcome before marking the PR ready.