fix(analytics): normalize seasonality month axis

tests/test_analysis_period_linkage.py

@@ -14,7 +14,7 @@ from routes.monthly_routes import (
     _resolve_monthly_period,
     get_monthly_summary_data,
 )
-from routes.sales_routes import _filter_growth_payload
+from routes.sales_routes import _filter_growth_payload, _sorted_valid_month_labels
 from services.analysis_period_service import make_analysis_period, make_month_period
 
 
@@ -250,6 +250,17 @@ def test_growth_period_slices_every_series_and_recomputes_period_kpis():
     assert period["end_month"] == "2026-05"
 
 
+def test_sales_seasonality_month_axis_ignores_nan_and_invalid_values():
+    assert _sorted_valid_month_labels([
+        "2026-04",
+        float("nan"),
+        None,
+        "invalid",
+        "2025-12",
+        "2026-04-30",
+    ]) == ["2025-12", "2026-04"]
+
+
 def test_monthly_frontend_has_replayable_period_and_no_fixed_years():
     script = (ROOT / "web/static/js/page-monthly-summary.js").read_text(encoding="utf-8")
     route = (ROOT / "routes/monthly_routes.py").read_text(encoding="utf-8")