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/httpserver/router.go

@@ -4,6 +4,7 @@
 package httpserver
 
 import (
+	"fmt"
 	"io/fs"
 	"net/http"
 
@@ -21,7 +22,7 @@ type Deps struct {
 	AgentAssetDir string // directory containing vetting-agent-linux-amd64; "" disables /assets
 }
 
-func NewRouter(d Deps) http.Handler {
+func NewRouter(d Deps) (http.Handler, error) {
 	r := chi.NewRouter()
 	r.Use(middleware.RealIP)
 	r.Use(middleware.Recoverer)
@@ -29,7 +30,7 @@ func NewRouter(d Deps) http.Handler {
 
 	staticFS, err := fs.Sub(web.Static, "static")
 	if err != nil {
-		panic(err)
+		return nil, fmt.Errorf("extract static assets: %w", err)
 	}
 	r.Handle("/static/*", http.StripPrefix("/static/", http.FileServer(http.FS(staticFS))))
 
@@ -80,5 +81,5 @@ func NewRouter(d Deps) http.Handler {
 	r.Get("/register/quick.sh", d.UI.QuickRegisterScript)
 	r.Get("/events", d.UI.SSE)
 
-	return r
+	return r, nil
 }