feat: refresh homepage as unified gateway landing

[addxiaoyi]

Summary

• rebuild the homepage into a WanAPIs-style unified AI API gateway landing page
• add a dedicated /en homepage route and keep locale switching aligned with /home and /en
• update the public entry flow so backend mode still allows the homepage routes

Verification

• npm run typecheck
• npm run build
• local preview responded on /home and /en

[github-actions]

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