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/imageref/imageref.go

@@ -30,6 +30,12 @@ func Parse(ref string) Ref {
 	return r
 }
 
+// IsRegistryHost reports whether the first path segment of a repository names
+// a registry: it contains a "." or ":" or is "localhost".
+func IsRegistryHost(segment string) bool {
+	return strings.ContainsAny(segment, ".:") || segment == "localhost"
+}
+
 // String reassembles the reference as repo[:tag][@digest].
 func (r Ref) String() string {
 	s := r.Repo