fix: add bounty list loading state

[tristanwork]

Summary

• add a bounty list loading controller that shows a spinner while the API request is pending
• hide the spinner after both successful and failed requests
• render a friendly error state when loading fails
• add accessible loading spinner markup with status/aria-live semantics

Testing

• npm test

Fixes #18

[tristanwork]

Local verification passes on this branch:

npm test
# parser tests: 6 passed
# bounty list loading tests: 10 passed

The GitHub Actions check is currently action_required with no jobs started, so it appears to need maintainer approval to run for this fork PR.