feat(freshness): enhance diagnostics and update snapshot structure

- Introduced a new Diagnostics struct to capture transaction visibility state and activity state.
- Updated BuildSnapshot function to return diagnostics alongside snapshot, completeness, and sampling.
- Enhanced test cases to validate the new diagnostics data.
- Updated frontend components to utilize the new diagnostics information for improved user feedback on freshness context.

This change improves the observability of transaction activity and enhances the user experience by providing clearer insights into the freshness of data.

backend/api/track1/bridge_status_data.go

@@ -132,7 +132,7 @@ func (s *Server) BuildBridgeStatusData(ctx context.Context) map[string]interface
 		data["operator_verify"] = ov
 	}
 	if s.freshnessLoader != nil {
-		if snapshot, completeness, sampling, err := s.freshnessLoader(ctx); err == nil && snapshot != nil {
+		if snapshot, completeness, sampling, diagnostics, err := s.freshnessLoader(ctx); err == nil && snapshot != nil {
 			subsystems := map[string]interface{}{
 				"rpc_head": map[string]interface{}{
 					"status":       chainStatusFromProbe(p138),
@@ -194,6 +194,9 @@ func (s *Server) BuildBridgeStatusData(ctx context.Context) map[string]interface
 			data["freshness"] = snapshot
 			data["subsystems"] = subsystems
 			data["sampling"] = sampling
+			if diagnostics != nil {
+				data["diagnostics"] = diagnostics
+			}
 			data["mode"] = map[string]interface{}{
 				"kind":        modeKind,
 				"updated_at":  now,