webui: replace nautctl web server with a loadable plugin

Drop the web UI that was compiled into nautctl and serve the
torrent-ui front end (../torrent-ui/public) from a native plugin
(plugins/webui) loaded via `nautd --plugin`. The plugin talks to the
engine only through the host call_rpc ABI and adapts the daemon's RPC
surface to the qBittorrent-style contract the UI expects (snapshot/SSE,
torrent detail tabs, add/delete, cookie auth).

Also folds in the daemon refactor that owns per-torrent worker threads
and the swarm engine (naut_swarm) used by the plugin's data source.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -8,3 +8,10 @@ compile_commands.json
 
 # Test scratch
 /tmp/
+
+# Local downloads and packaging scratch
+/downloads/
+/package/
+/package.zip
+# Stray torrents dropped at the repo root (fixtures under tests/ stay tracked)
+/*.torrent