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/pipeline.templ

@@ -223,32 +223,11 @@ func stageStateByName(name string) (model.RunState, bool) {
 	return s, ok
 }
 
-// stageDuration renders node timing as "1.2s" / "12s" / "4m". Empty
-// string when the node hasn't started or hasn't finished.
 func stageDuration(n PipelineNode) string {
-	if n.StartedAt == nil {
-		return ""
-	}
-	end := time.Now()
-	if n.CompletedAt != nil {
-		end = *n.CompletedAt
-	}
-	d := end.Sub(*n.StartedAt)
-	if d < 0 {
-		d = 0
-	}
-	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:
-		return fmt.Sprintf("%dm", int(d/time.Minute))
-	default:
-		return fmt.Sprintf("%dh", int(d/time.Hour))
+	if d := elapsed(n.StartedAt, n.CompletedAt); d >= 0 {
+		return fmtElapsed(d, false)
 	}
+	return ""
 }
 
 // stageDisplayName turns the internal single-word state/stage identifier