feat: use relative URLs for frontend API calls

[cfsmp3]

Summary

• Change frontend to use relative URLs (/) instead of hardcoded localhost values
• Add getWebSocketURL() helper to construct WebSocket URLs from current host
• Derive sync server URL (containerOrigin) from window.location.origin
• Remove unused VITE_* environment variables from jest config

This enables the same Docker image to work in any environment since nginx handles routing to the backend. This is a prerequisite for automated deployment where images should be environment-agnostic.

Changes

File	Change
URLs.ts	Use relative URLs, add getWebSocketURL() helper
HomePage.tsx	Use new getWebSocketURL() for WebSocket connection
HomePage.test.tsx	Add getWebSocketURL to mock
jest.config.cjs	Remove unused VITE_* env vars
Hero.test.tsx.snap	Updated snapshot (URL: /auth/oauth)

Test plan

• All 670 tests pass
• Docker image builds successfully
• Container runs and serves frontend correctly
• Test on VPS with nginx reverse proxy

🤖 Generated with Claude Code

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