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/host_page.templ

@@ -283,9 +283,6 @@ func profileChipValue(p string) string {
 	return p
 }
 
-// runDuration formats the elapsed time for a run using the same buckets
-// as stageDuration. In-flight runs clock from StartedAt to now so the
-// run-page header + runs-table row keep ticking on each SSE push.
 func runDuration(r *model.Run) string {
 	if r == nil || r.StartedAt.IsZero() {
 		return ""
@@ -298,18 +295,7 @@ func runDuration(r *model.Run) string {
 	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 %ds", int(d/time.Minute), int((d%time.Minute)/time.Second))
-	default:
-		return fmt.Sprintf("%dh %dm", int(d/time.Hour), int((d%time.Hour)/time.Minute))
-	}
+	return fmtElapsed(d, true)
 }
 
 // stageForName returns the persisted Stage row for a given name, or a