Add dynamic DNS and show the running kernel on the overview

router.ddns keeps provider records on the uplink's addresses with ddclient,
run by a timer and by a watcher that starts it when an address changes. The
Dynamic DNS page edits it; the secret is a private file on the router. The
overview shows the running kernel and says when the current system boots a
different one. PLAN.md records why the A/B image approach was not pursued.

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

pkgs/router-webui/collect.go

@@ -16,6 +16,7 @@ import (
 	"net/netip"
 	"os"
 	"path/filepath"
+	"regexp"
 	"slices"
 	"strconv"
 	"strings"
@@ -67,6 +68,8 @@ type UplinkStatus struct {
 type SystemStatus struct {
 	Uptime      time.Duration
 	Load        string
+	Kernel      string  // the running kernel
+	KernelNext  string  // the kernel of the current system when it differs: a reboot is due
 	CPU         float64 // percent; NaN until the second sample
 	MemUsed     uint64
 	MemTotal    uint64
@@ -253,6 +256,12 @@ func systemStatus() SystemStatus {
 			s.Uptime = time.Duration(secs) * time.Second
 		}
 	}
+	if data, err := os.ReadFile("/proc/sys/kernel/osrelease"); err == nil {
+		s.Kernel = strings.TrimSpace(string(data))
+	}
+	if dest, err := filepath.EvalSymlinks("/run/current-system/kernel"); err == nil {
+		s.KernelNext = pendingKernel(s.Kernel, dest)
+	}
 	if data, err := os.ReadFile("/proc/loadavg"); err == nil {
 		if f := strings.Fields(string(data)); len(f) >= 3 {
 			s.Load = strings.Join(f[:3], " ")
@@ -282,6 +291,18 @@ func systemStatus() SystemStatus {
 	return s
 }
 
+var kernelDirRe = regexp.MustCompile(`-linux-([0-9][^/]*)/`)
+
+// pendingKernel is the version of the kernel image the current system boots
+// (a path like /nix/store/…-linux-6.12.5/bzImage) if it is not the running one.
+func pendingKernel(running, image string) string {
+	m := kernelDirRe.FindStringSubmatch(image)
+	if m == nil || running == "" || m[1] == running || strings.HasPrefix(running, m[1]+"-") {
+		return ""
+	}
+	return m[1]
+}
+
 // Sensor is one temperature the machine reports.
 type Sensor struct {
 	Key   string // stable name of its series