Add signed updates from a bundle or a URL

router-release signs a release as a binary cache; router-update verifies
it against the router's update keys, migrates settings.json, and applies
it with commit-confirm. Includes the web UI page and a VM test.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/system.go

@@ -18,6 +18,8 @@ import (
 type System struct {
 	ConfigDir string // the configuration flake, settings.json in it
 	StateDir  string // /var/lib/router: secrets, UI state
+	// ReleaseFile describes the installed nixos-router; /etc/router-release if empty.
+	ReleaseFile string
 }
 
 func (s System) settingsPath() string  { return filepath.Join(s.ConfigDir, "settings.json") }