feat: Voxlen API proxy + token-based desktop auth #4760

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/voxlen/pull/41

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Summary

• Add Vercel serverless endpoints at voxlen.ai/api/*:
  • /api/me — verify Google session token, return plan + features
  • /api/stt — proxy audio to Deepgram Nova-3 (no user API key needed)
  • /api/grammar — proxy text to Anthropic Claude for grammar correction
  • /api/deepgram-token — issue short-lived Deepgram temp keys for streaming
• Shared _auth.ts module verifies Google access tokens via /oauth2/v3/userinfo
• Admin (ccantynz@gmail.com) gets full feature access; free users get STT only
• Desktop app SettingsPanel verifies session token against /api/me and routes all AI calls through proxy when token is present
• Dashboard shows copyable session token with instructions to paste in Settings → Voxlen Account
• Update vercel.json rewrites so /api/* passes to serverless functions

Test plan

• Sign in at voxlen.ai with admin Google account
• Copy session token from Dashboard → Connect Desktop App
• Paste token in desktop Settings → Voxlen Account → Connect
• Verify dictation works without any API keys entered
• Verify grammar correction works via proxy
• Verify free account only gets STT (no grammar without upgrade)

https://claude.ai/code/session_011MFNHeFKrhfuSrEYVgrdyr EOF )

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