docs: Add API auth example and troubleshooting section

[zhuamber370]

What

Add a short troubleshooting section for common local dev issues:

• Backend 401 when AFKMS_REQUIRE_AUTH=true
• Where to set KMS_API_KEY / NEXT_PUBLIC_API_KEY
• Confirming API base (NEXT_PUBLIC_API_BASE)

Where to start

• docs/contributing/dev-setup.md
• Cross-link to backend/README.md and frontend/README.md where relevant

Why

Reduces setup friction and improves time-to-first-contribution.

Acceptance criteria

• Troubleshooting section added
• Steps are copy-paste runnable
• No secrets in docs

Verification

• Docs-only. Optionally validate locally by toggling auth and confirming the frontend can call the API.