docs: add frontend console screenshot and usage guide

[stackByStack]

This PR improves the Frontend Console section in docs/api.md by documenting the local server_v2 web console with a screenshot and a practical usage walkthrough.

Changes

• add a named screenshot asset for the server_v2 web console
• embed the screenshot in docs/api.md
• expand the Frontend Console section with:
  • a quick layout overview of the left, center, and right rails
  • a typical local workflow for connecting to the backend, selecting a managed runtime profile, submitting a job, and inspecting results
  • guidance on where to look when debugging requests, capabilities, and runtime behavior
• add a .gitignore exception so PNG assets under docs/assets/ can be tracked

Notes

• documentation and asset update only
• no runtime or API behavior changes
• no tests were run for this change