fix(master): review fixes for the Ledger UI (#18)

- icons/gen writes to the current directory so `go generate
  ./internal/master/web/icons` works (output verified byte-identical)
- share the registry-host check as imageref.IsRegistryHost (with test)
- agents fragment refreshes the "N machines" header count out of band
- ignore the saved machine filter at <=760px where the machine list is hidden
- aria-current on the active machine filter link, aria-pressed on theme toggle
- CLAUDE.md: icons package, SSE ping event, swarm_cluster_id

Claude-Session: https://claude.ai/code/session_01GAZsp3P7Qto6JnhipxWUJQ

internal/master/web/server.go

@@ -559,7 +559,11 @@ func (s *Server) agentsFragment(w http.ResponseWriter, r *http.Request) {
 		s.fail(w, err)
 		return
 	}
-	s.render(w, http.StatusOK, "agents.html", "agents", data)
+	if data.Nav, err = s.navData(r.Context(), "agents"); err != nil {
+		s.fail(w, err)
+		return
+	}
+	s.render(w, http.StatusOK, "agents.html", "agents-fragment", data)
 }
 
 // agentConnectFragment is the last step of the "add agent" panel: it says