ui: per-host Jobs sub-tab; drop unused Settings stub

Adds /hosts/{id}/jobs page listing recent jobs for the host (newest
first, capped at 100) with click-through to /jobs/{id}. Converts the
Jobs placeholder <div> to a real <a> nav link; removes the Settings
stub entirely. Also registers durationHuman template func and a
.jobs-row CSS grid to match the existing .schd-row idiom.

internal/server/ui/funcs.go

@@ -75,6 +75,28 @@ func funcMap() template.FuncMap {
 			return *p
 		},
 		"sub": func(a, b int) int { return a - b },
+		// durationHuman formats the elapsed time between two *time.Time
+		// values as a short human string: "350ms", "4.2s", "2m 15s",
+		// "1h 4m". Returns "—" when either pointer is nil.
+		"durationHuman": func(start, end *time.Time) string {
+			if start == nil || end == nil {
+				return "—"
+			}
+			d := end.Sub(*start)
+			if d < 0 {
+				d = -d
+			}
+			if d < time.Second {
+				return fmt.Sprintf("%dms", d.Milliseconds())
+			}
+			if d < time.Minute {
+				return fmt.Sprintf("%.1fs", d.Seconds())
+			}
+			if d < time.Hour {
+				return fmt.Sprintf("%dm %ds", int(d.Minutes()), int(d.Seconds())%60)
+			}
+			return fmt.Sprintf("%dh %dm", int(d.Hours()), int(d.Minutes())%60)
+		},
 		// joinComma joins a slice with ", ". Used by the schedule list
 		// to render retention summaries.
 		"joinComma": func(parts []string) string { return strings.Join(parts, ", ") },