Add Simplified Chinese localization for core UI

[yaojingang]

What changed

• Localized the core self-hosted OpenSEO interface in Simplified Chinese.
• Covered navigation, onboarding, dashboard, projects, keyword research, saved keywords, rank tracking, domain overview, backlinks, site audits, AI visibility, Search Console, Analytics, AI and MCP, settings, support, tables, filters, exports, errors, and empty states.
• Set the document language to zh-CN and display country names in Simplified Chinese.
• Added support for Chinese comma and ideographic comma separators in keyword and filter inputs while preserving existing separators.
• Kept product and technical names such as DataForSEO, MCP, GSC, GA4, SERP, CPC, KD, and Lighthouse unchanged.

Why

The current self-hosted experience has no Simplified Chinese interface. This draft PR provides a tested proof of concept and a consistent Chinese SEO glossary across the main product workflows.

This implementation directly changes the core UI language to Simplified Chinese. Hosted authentication, subscription, and billing surfaces remain outside this proof-of-concept scope. The translated copy can move into locale resources if the project adopts language switching.

User impact

Simplified Chinese users can complete the main self-hosted workflows without relying on browser translation. Chinese punctuation also works in multi-value search and filter fields.

Validation

• pnpm test: 123 test files and 1,008 tests passed
• pnpm lint: 0 warnings and 0 errors
• pnpm build: production client, SSR, and TypeScript checks passed
• Browser verification: 11 core routes checked at 1440px desktop and 375px mobile widths
• Docker verification: localized image started successfully and reported a healthy status