fix: translation + grammar proxy bugs for Voxlen account users #4751

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/voxlen/pull/50

Summary

Three bugs affecting users who connect via Voxlen account key (no BYOK):

• Translation broken for Voxlen account users — translate_text in Rust called Anthropic directly using api_key from grammar config, which is None when a Voxlen account key is set. Now checks voxlen_api_key first and proxies through new /api/translate endpoint.
• New /api/translate serverless endpoint — mirrors the grammar endpoint pattern: verifies Voxlen token, then calls Claude Haiku for translation. Supports all 20 languages already in the desktop app.
• preserve_tone ignored in grammar proxy — Rust client sends preserve_tone (snake_case) but server only read preserveTone (camelCase). Fixed to accept both.
• Last useGoogleLogin direct call replaced with useGoogleSignIn shared hook in SEOPage.

Test plan

• Translation feature works when connected via Voxlen account key (no Anthropic key set)
• Grammar preserve-tone option takes effect in proxy mode
• SEO pages still show Google sign-in button

https://claude.ai/code/session_011MFNHeFKrhfuSrEYVgrdyr

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