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>

tests/fixtures/phase7.lua

@@ -1,5 +1,5 @@
 function on_torrent_finished(event)
-    naut.move_file(event.torrent_id, 0, "/tmp/naut-phase7-finished")
+    print("torrent " .. event.torrent_id .. " finished")
 end
 
 function on_file_complete(event)