refactor(i18n): ask the catalogue for a sentence in one helper

`t()` answers a missing key with the key, so "does this code have a
sentence" was a comparison written inline at every site that wanted a
code's sentence or a fallback, twenty-odd times in several spellings.
translateOrNull, translateCodeOrNull and translateCode in
src/lib/i18n/translate-code.ts now hold that probe, every site uses
them, and a guard refuses the inline comparison anywhere else.

src/components/charts/health-chart.tsx

@@ -89,6 +89,7 @@ import {
   DEFAULT_CHART_RANGE,
   rangeWindowDays,
 } from "./chart-range-tabs";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 // The range tabs (7 / 30 / 90 / All, a calendar-day window ending now) and
 // the "All" tab's ten-year fetch span live with the shared
@@ -399,8 +400,7 @@ function getTypeLabel(
   const key = BASE_TYPE_LABEL_KEYS[type];
   if (key) return t(key);
   const measurementKey = measurementLabelKey(type);
-  const label = t(measurementKey);
-  return label === measurementKey ? type : label;
+  return translateOrNull(t, measurementKey) ?? type;
 }
 
 /**