fix custom-agent UI when agents API is disabled

[Eilen6316]

Summary

• add an explicit agents API status endpoint for the custom-agent UI
• disable custom-agent creation and listing actions when agents_api.enabled is off
• show the exact config.yaml snippet needed to enable the protected HTTP API
• document the requirement in the setup section of the README

Fixes #2388

Related to #1482, #1518, #2378

Testing

• verified frontend formatting for the touched files
• validated backend status route and custom-agent API disabled tests in code changes

[Eilen6316]

Added a follow-up fix for the failing E2E check.

Root cause: this PR introduced a new frontend request to /api/agents/status, but the Playwright E2E mock helper only mocked /api/agents and /api/agents/*, so the agent gallery path could not enter the normal enabled state during tests.

Fix: mocked /api/agents/status in frontend/tests/e2e/utils/mock-api.ts and pushed the update in commit b7e33b1.