Thread archiving support

[BunsDev]

Summary

Add thread archiving as a distinct action from deletion. Currently, threads can only be hard-deleted via deletedAt. An archive mechanism would allow users to hide threads from the sidebar without permanently removing them.

Acceptance criteria

• Archive/unarchive action in thread context menu
• Archived threads hidden from sidebar by default
• "Show archived threads" toggle in sidebar
• Data model distinguishes archived vs. deleted (e.g. archivedAt timestamp)
• Backend WebSocket command support for archive/unarchive

Context

From TODO.md — "Thread archiving". Triaged as deferred (not release-blocking for v0.0.2).

🤖 Filed by Claude Code