feat(i18n): add Chinese (zh) locale for web UI

[sunriseydy]

Add Simplified Chinese translation for the web interface.

What's included:

• Translated 303 UI strings covering all sections: navigation, dashboard, subscriptions, forms, settings, calendar, analytics, auth, schedules, and themes
• lang.name set to 中文
• Proper-noun service names kept in English per locale README guidelines (SMTP, Pushover, Webhook, SubTrackr, API, JSON, CSV, iCal, MCP, etc.)
• api_docs.* section kept in English (developer-facing reference)

Verification:

• Valid JSON (python3 -c "import json; json.load(open('web/locales/zh.json'))")
• All 346 keys present, matching en.json key set
• npx playwright test tests/i18n-coverage.spec.js --workers=1 — pending CI check

[bscott]

Thanks @sunriseydy I'll review as I'm working on the v0.6.6 release and will try to get this in.