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/events/events.go

@@ -22,6 +22,11 @@ type subscriber struct {
 	ch chan Event
 }
 
+const (
+	defaultSubscriberBuffer = 32
+	heartbeatInterval       = 15 * time.Second
+)
+
 // Hub is an in-process fan-out for SSE subscribers.
 type Hub struct {
 	mu        sync.RWMutex
@@ -29,13 +34,16 @@ type Hub struct {
 	subs      map[int64]*subscriber
 	buffer    int
 	heartbeat time.Duration
+	done      chan struct{}
+	closeOnce sync.Once
 }
 
 func NewHub() *Hub {
 	h := &Hub{
 		subs:      map[int64]*subscriber{},
-		buffer:    32,
-		heartbeat: 15 * time.Second,
+		buffer:    defaultSubscriberBuffer,
+		heartbeat: heartbeatInterval,
+		done:      make(chan struct{}),
 	}
 	go h.heartbeatLoop()
 	return h
@@ -70,11 +78,16 @@ func (h *Hub) Subscribe() (id int64, ch <-chan Event, cancel func()) {
 func (h *Hub) heartbeatLoop() {
 	t := time.NewTicker(h.heartbeat)
 	defer t.Stop()
-	for range t.C {
-		h.Publish(Event{
-			Name:    "heartbeat",
-			Payload: fmt.Sprintf(`<span data-heartbeat="%d"></span>`, time.Now().Unix()),
-		})
+	for {
+		select {
+		case <-h.done:
+			return
+		case <-t.C:
+			h.Publish(Event{
+				Name:    "heartbeat",
+				Payload: fmt.Sprintf(`<span data-heartbeat="%d"></span>`, time.Now().Unix()),
+			})
+		}
 	}
 }
 
@@ -140,5 +153,16 @@ func splitLines(s string) []string {
 	return out
 }
 
-// Shutdown is a no-op placeholder wired into graceful shutdown.
-func (h *Hub) Shutdown(_ context.Context) error { return nil }
+// Shutdown stops the heartbeat goroutine and closes all subscriber channels.
+func (h *Hub) Shutdown(_ context.Context) error {
+	h.closeOnce.Do(func() {
+		close(h.done)
+		h.mu.Lock()
+		for id, s := range h.subs {
+			close(s.ch)
+			delete(h.subs, id)
+		}
+		h.mu.Unlock()
+	})
+	return nil
+}