fix: resolve ERR_INSUFFICIENT_RESOURCES connection exhaustion

- Add concurrency limiter (max 4 GET requests) to API layer, leaving
  slots for SSE and health checks. Write ops bypass the limiter.
- Add AbortController to ContainerStats, project detail page, and
  dashboard to cancel in-flight requests on navigation/unmount.
- Move global SSE connection from layout to events page (only consumer).
- Add 30s heartbeat to SSE endpoint to detect zombie connections.
- Serialize dashboard project fetches to avoid parallel burst.
- Rebuild frontend in dev-server.sh so go:embed stays in sync.

internal/api/sse.go

@@ -7,6 +7,7 @@ import (
 	"log/slog"
 	"net/http"
 	"strings"
+	"time"
 
 	"github.com/go-chi/chi/v5"
 
@@ -158,6 +159,10 @@ func (s *Server) streamEvents(w http.ResponseWriter, r *http.Request) {
 	})
 	defer s.eventBus.Unsubscribe(sub)
 
+	// Periodic heartbeat so the browser detects dead connections.
+	heartbeat := time.NewTicker(30 * time.Second)
+	defer heartbeat.Stop()
+
 	ctx := r.Context()
 	for {
 		select {
@@ -168,6 +173,10 @@ func (s *Server) streamEvents(w http.ResponseWriter, r *http.Request) {
 				return
 			}
 			writeSSE(w, flusher, evt)
+		case <-heartbeat.C:
+			// SSE comment line — keeps the connection alive without triggering onmessage.
+			fmt.Fprintf(w, ": heartbeat\n\n")
+			flusher.Flush()
 		}
 	}
 }