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/rest/stats_internal_test.go

@@ -76,6 +76,13 @@ func TestLoadExplorerStatsReturnsValues(t *testing.T) {
 				case 11:
 					*dest[0].(*int64) = 40
 					*dest[1].(*time.Time) = time.Now().Add(-5 * time.Second)
+				case 12:
+					*dest[0].(*int64) = 42
+					*dest[1].(*time.Time) = time.Now().Add(-3 * time.Second)
+				case 13:
+					*dest[0].(*int64) = 128
+					*dest[1].(*int64) = 10
+					*dest[2].(*int64) = 22
 				default:
 					t.Fatalf("unexpected query call %d", call)
 				}
@@ -102,6 +109,8 @@ func TestLoadExplorerStatsReturnsValues(t *testing.T) {
 	require.NotNil(t, stats.Freshness.ChainHead.BlockNumber)
 	require.Equal(t, int64(40), *stats.Freshness.LatestIndexedTransaction.BlockNumber)
 	require.Equal(t, int64(4), *stats.Freshness.LatestNonEmptyBlock.DistanceFromHead)
+	require.Equal(t, "active", stats.Diagnostics.ActivityState)
+	require.Equal(t, int64(4), *stats.Diagnostics.TxLagBlocks)
 	require.Equal(t, "reported", string(stats.Freshness.ChainHead.Source))
 	require.Equal(t, freshness.CompletenessComplete, stats.Completeness.GasMetrics)
 	require.Equal(t, freshness.CompletenessComplete, stats.Completeness.UtilizationMetric)