fix(dashboard): keep rewrite API credentials server-side

[mameikagou]

Summary

• default dashboard project requests to the existing /api/openmemory server proxy
• stop baking API URLs and keys into the browser-facing Docker build environment
• add regression coverage for the proxy fallback and Docker configuration

This keeps OPENMEMORY_API_KEY on the server side while preserving an explicit NEXT_PUBLIC_API_URL override for deployments that intentionally use one.

Validation

• node --test dashboard/tests/auth-config.test.js
• cd dashboard && node --test tests/auth-config.test.js
• cd dashboard && npm run build
• git diff --check