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

modules/settings/upgrade.nix

@@ -41,7 +41,7 @@ let
     ];
     text = ''
       usage() {
-        echo "usage: router-upgrade check | preview [--force] | download | apply [timeout]" >&2
+        echo "usage: router-upgrade check | preview [--force] | download | apply [timeout] | --request" >&2
         echo "  check     the newest revision of router.update.channel, against the running one" >&2
         echo "  preview   the packages that change and the size of the download (nothing is fetched)" >&2
         echo "  download  fetch and build what was previewed, without activating it" >&2
@@ -50,6 +50,23 @@ let
       }
       die() { echo "router-upgrade: $*" >&2; exit 1; }
       [ $# -gt 0 ] || usage
+      if [ "$1" = --request ]; then
+        # What the web UI asked for (${dir}/request.json).
+        req=${dir}/request.json
+        [ -r "$req" ] || die "no request"
+        action=$(jq -er .action "$req")
+        force=$(jq -r 'if .force then "--force" else "" end' "$req")
+        timeout=$(jq -r '.timeout // ""' "$req")
+        rm -f "$req"
+        # The web UI shows the last run (a finished unit forgets its journal).
+        exec > >(tee ${dir}/last.log) 2>&1
+        case $action in
+          preview) set -- preview ''${force:+"$force"} ;;
+          apply) set -- apply ''${timeout:+"$timeout"} ;;
+          check | download) set -- "$action" ;;
+          *) die "unknown request: $action" ;;
+        esac
+      fi
       cmd=$1
       shift
       [ "$(id -u)" -eq 0 ] || die "must run as root"
@@ -234,5 +251,22 @@ in
     };
   };
 
-  config.environment.systemPackages = [ upgrade ];
+  config = {
+    environment.systemPackages = [ upgrade ];
+
+    # The web UI starts this rather than running anything itself, so a switch
+    # that restarts the UI does not cut an upgrade short; a switch must not
+    # restart or stop it either (router-apply is what runs the switch).
+    systemd.services.router-upgrade = {
+      description = "Look for, prepare or apply a nixpkgs upgrade";
+      restartIfChanged = false;
+      stopIfChanged = false;
+      path = [ "/run/current-system/sw" ];
+      serviceConfig = {
+        Type = "oneshot";
+        ExecStart = "${lib.getExe upgrade} --request";
+        TimeoutStartSec = "2h";
+      };
+    };
+  };
 }