chore: cleanup sprint — dead CSS, dedup helpers, handler refactor

Remove ~126 lines of orphaned CSS from tile slim-down and old detail
layout. Consolidate 4 duplicate duration formatters into shared
elapsed()/fmtElapsed() helpers. Break 160-line Result handler into
focused sub-functions. Implement real Hub.Shutdown() (was a no-op).
Standardize agent error responses to JSON. Replace panic() in router
init with error return. Extract magic numbers as named constants.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/templates/helpers.go

@@ -0,0 +1,42 @@
+package templates
+
+import (
+	"fmt"
+	"time"
+)
+
+func elapsed(start, end *time.Time) time.Duration {
+	if start == nil {
+		return -1
+	}
+	e := time.Now()
+	if end != nil {
+		e = *end
+	}
+	d := e.Sub(*start)
+	if d < 0 {
+		return 0
+	}
+	return d
+}
+
+func fmtElapsed(d time.Duration, long bool) string {
+	switch {
+	case d < time.Second:
+		return fmt.Sprintf("%dms", int(d/time.Millisecond))
+	case d < 10*time.Second:
+		return fmt.Sprintf("%.1fs", d.Seconds())
+	case d < time.Minute:
+		return fmt.Sprintf("%ds", int(d/time.Second))
+	case d < time.Hour:
+		if long {
+			return fmt.Sprintf("%dm %ds", int(d/time.Minute), int((d%time.Minute)/time.Second))
+		}
+		return fmt.Sprintf("%dm", int(d/time.Minute))
+	default:
+		if long {
+			return fmt.Sprintf("%dh %dm", int(d/time.Hour), int((d%time.Hour)/time.Minute))
+		}
+		return fmt.Sprintf("%dh", int(d/time.Hour))
+	}
+}