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>

cmd/vetting/main.go

@@ -208,12 +208,15 @@ func main() {
 		ui.PXE = supervisor
 	}
 
-	router := httpserver.NewRouter(httpserver.Deps{
+	router, err := httpserver.NewRouter(httpserver.Deps{
 		UI:            ui,
 		Agent:         agentAPI,
 		LiveDir:       cfg.PXE.LiveDir,
 		AgentAssetDir: cfg.Agent.AssetDir,
 	})
+	if err != nil {
+		log.Fatalf("router: %v", err)
+	}
 
 	srv := &http.Server{
 		Addr:              cfg.Server.Bind,