fleet-manager: chrony tracking metrics relative to the NTP source

The agent only reported chrony as synced|unsynced — nothing about how well the
clock tracks. Parse `chronyc tracking` for the source-relative metrics.

- proto: HealthSnapshot.chrony_offset_seconds (16, signed system-time offset,
  fast=+), chrony_source (17, reference name/IP), chrony_stratum (18); regenerated.
- agent: replace chronyState() with chronyTracking()/parseChrony() — one exec,
  parses Leap status + Reference ID + Stratum + System time offset (+ test).
- manager: Health.{ChronyOffset,ChronySource,ChronyStratum}; snapshotToHealth maps
  them; influx writes chrony_offset_seconds/_stratum/_source and whitelists the
  offset + stratum as graphable fields.
- UI: chronySummary() + offsetMs() — the chrony row now shows
  "synced · 192.168.50.10 · s3 · +0.01ms" instead of just "synced".

serving/fleet-manager/influx.go

@@ -26,7 +26,7 @@ const influxMeasurement = "fleet_health"
 var metricFields = map[string]bool{
 	"load_avg": true, "cpu_pct": true, "temp_c": true, "mem_used_pct": true,
 	"tmpfs_used_pct": true, "chromium_crashes": true, "kiosk_active": true,
-	"uptime_seconds": true,
+	"uptime_seconds": true, "chrony_offset_seconds": true, "chrony_stratum": true,
 }
 
 var serialRe = regexp.MustCompile(`^[0-9a-fA-F]+$`) // cert serials are hex
@@ -74,14 +74,17 @@ func snapshotPoint(serial, hostnqn, starship string, snap *fleetpb.HealthSnapsho
 		tags["starship"] = starship
 	}
 	fields := map[string]any{
-		"load_avg":         snap.GetLoadAvg(),
-		"cpu_pct":          snap.GetCpuPct(),
-		"temp_c":           snap.GetTempC(),
-		"mem_used_pct":     snap.GetMemUsedPct(),
-		"tmpfs_used_pct":   snap.GetTmpfsUsedPct(),
-		"chromium_crashes": int64(snap.GetChromiumCrashes()),
-		"kiosk_active":     boolToInt(snap.GetKioskActive()),
-		"uptime_seconds":   snap.GetUptimeSeconds(),
+		"load_avg":              snap.GetLoadAvg(),
+		"cpu_pct":               snap.GetCpuPct(),
+		"temp_c":                snap.GetTempC(),
+		"mem_used_pct":          snap.GetMemUsedPct(),
+		"tmpfs_used_pct":        snap.GetTmpfsUsedPct(),
+		"chromium_crashes":      int64(snap.GetChromiumCrashes()),
+		"kiosk_active":          boolToInt(snap.GetKioskActive()),
+		"uptime_seconds":        snap.GetUptimeSeconds(),
+		"chrony_offset_seconds": snap.GetChronyOffsetSeconds(),
+		"chrony_stratum":        int64(snap.GetChronyStratum()),
+		"chrony_source":         snap.GetChronySource(),
 		// string status fields kept as fields too (handy for annotating charts)
 		"nvme_root": snap.GetNvmeRoot(),
 		"auth":      snap.GetAuth(),