feat(downloads): ordered preferred_methods list honored for web tasks

The agent ignored its config.toml method preference for web-driven downloads
(only the local `unarr download` command read it), and resolveMethod tried
torrent first in auto mode — so a 'debrid only' user still got torrent tasks.

- config: preferred_methods (ordered list, e.g. ["debrid","usenet"]) with
  MethodOrder() resolution; back-compat with the singular preferred_method.
  Methods absent from the list are disabled (debrid-only never torrents).
- resolveMethod/tryFallback honor the config order (gating, no fallback to a
  method outside the list) over the per-task preference.
- report preferred_methods on register so the web honors it (resolves debrid,
  gates the P2P stream fallback).
- enable the usenet downloader when usenet is listed (it was never enabled).
- config_menu: ordered presets (debrid-only, debrid→torrent, debrid→usenet…).

Tests: resolveMethod gating + fallback within/outside the list.

internal/agent/daemon.go

@@ -41,6 +41,9 @@ type DaemonConfig struct {
 	HWDevices     []string // device files + driver bins detected at probe time
 	AutoUpgrade   bool     // honor server-flagged upgrades by downloading + restarting (default: true)
 	Downlink      string   // realtime downlink transport: "auto" (SSE+long-poll fallback) | "sse" | "poll"
+	// PreferredMethods is the ordered download-method preference from config.toml
+	// (e.g. ["debrid","usenet"]). Reported to the web so it honours the gating.
+	PreferredMethods []string
 }
 
 // Daemon manages agent registration and the sync loop.
@@ -165,6 +168,7 @@ func (d *Daemon) Register(ctx context.Context) error {
 		VPNServer:          d.vpnServer,
 		FunnelURL:          d.funnelURL,
 		IsDocker:           RunningInDocker(),
+		PreferredMethods:   d.cfg.PreferredMethods,
 	}
 	if free, total, err := DiskInfo(d.cfg.DownloadDir); err == nil {
 		req.DiskFreeBytes = free