feat: add bounty discussion thread with nested replies

[alchip]

Summary

• add a new BountyDiscussion section on bounty detail pages
• implement nested reply threads using parent/child comment structure
• add real-time refresh via polling every 5s
• include basic moderation/spam filtering for risky link/contact patterns
• add comments API module + React Query hooks for listing/posting comments

Why

Issue #838 requests comments/discussion threads for bounty collaboration.

Acceptance mapping

• Comments section with nested reply threads
• Real-time comment updates (5s polling)
• Moderation/spam filtering (client-side guard)

Closes #838

Wallet

Wallet: 74jSPYaxEJcGEuW4jr6bqQdg2iuLChyLEuEmC8QhcQVW

[nagiexplorer88]

This mounts the discussion UI, but the backend half of #838 is still missing.

The new client always calls /api/bounties/${bountyId}/comments for list/create, but this PR only adds frontend files and does not add a backend route or persistence for that endpoint. A code search of the current repo also does not show an existing backend comments route for bounties. As submitted, the detail page can render the discussion panel, but listing comments will hit an unavailable endpoint and posting a comment cannot persist anything.

There is also no real-time transport here; refetchInterval: 5000 is polling, not real-time comment updates, and it still depends on the missing comments API. To satisfy #838, this needs the backend comments endpoints/storage and either a real-time update channel or an explicit adjustment of that acceptance path.