Show estimated HbA1c in mmol/mol (IFCC) alongside the % value

Dutch lab results conventionally report HbA1c in mmol/mol rather than
DCCT/NGSP %. Adds gmi_mmol_per_mol() using the standard IFCC conversion
(Hoelzel et al. 2004) and shows it next to the existing % value on the
Samenvatting page and in the PDF summary.

Closes gitea #25.

tests/test_reporting.py

@@ -12,6 +12,7 @@ from reporting import (
     daily_gmi,
     day_of_week_pattern,
     detect_events,
+    gmi_mmol_per_mol,
     gmi_percent,
     gmi_trend,
     minutes_per_day,
@@ -35,6 +36,12 @@ def test_gmi_percent_matches_bergenstal_formula():
     assert gmi_percent(7.0) == pytest.approx(6.3, abs=0.05)
 
 
+def test_gmi_mmol_per_mol_matches_known_ifcc_reference_values():
+    # 6.5% is de bekende diagnostische drempel = 48 mmol/mol (IFCC).
+    assert gmi_mmol_per_mol(6.5) == 48
+    assert gmi_mmol_per_mol(7.0) == 53
+
+
 def test_daily_gmi_aggregates_per_calendar_day():
     glucose = _series(
         [