docs(support): add an API connection troubleshooting guide

[Mkalbani]

closes #378

Summary

• Adds a focused troubleshooting guide for common API connection failures.
• Exposes reusable guidance from the config layer so UI and docs can reference the same troubleshooting contract.
• Links the guide from the README and env example.

Changes

• Added docs/TROUBLESHOOTING.md with remedies for misconfigured API bases, CORS issues, timeouts, and 401/403 responses.
• Added a reusable troubleshooting guide export in src/lib/config.ts.
• Extended src/lib/tests/config.test.ts to cover the new guide contract.
• Linked the guide from README.md and .env.example.

Verification

• npm run lint
• npm test -- --runInBand src/lib/__tests__/config.test.ts
• npm test -- --runInBand
• npm run build