Add the Packages page for nixpkgs upgrades

A page under System for router-upgrade: the channel to follow, Check,
Preview (the package versions that change, the download size, what would be
built here, a reboot note for a new kernel), Download now and Upgrade. The
steps run in router-upgrade.service, which the page asks through a request
file; the last run's output is kept in upgrade/last.log because a finished
one-shot unit has no journal to look up.

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

pkgs/router-webui/system.go

@@ -21,6 +21,8 @@ type System struct {
 	StateDir  string // /var/lib/router: secrets, UI state
 	// ReleaseFile describes the installed nixos-router; /etc/router-release if empty.
 	ReleaseFile string
+	// VersionFile holds the running NixOS version; /run/current-system/nixos-version if empty.
+	VersionFile string
 	// LeasesDNSDir is where leases-dns leaves its status; /run/router-leases-dns if empty.
 	LeasesDNSDir string
 	ACMEDir      string // where the ACME client keeps the web UI's certificate (fullchain.pem, key.pem), if any