fix: converge the two factors a glucose reading was converted by

A value imported in mmol/L was multiplied by 18.016; one displayed in
mmol/L was divided by 18.0182. The two sat in different files, both
correct-looking, and a reading that went in as 5.3 did not come back out
as 5.3. Latent while nobody could select mmol/L, live the moment they can.

One exported constant now, on the guideline figure the display path
already used, and the glucose insight page takes it from there rather than
keeping a third copy. The test names the constant instead of repeating the
digits, which is how the two came apart in the first place, and it asserts
the round trip a reader on mmol/L actually experiences.

src/app/insights/blood-glucose/page.tsx

@@ -7,7 +7,7 @@ import { GlucoseClinicalPanel } from "@/components/insights/glucose/glucose-clin
 import { useAuth } from "@/hooks/use-auth";
 import { useModulePageGuard } from "@/hooks/use-module-page-guard";
 import { useTranslations } from "@/lib/i18n/context";
-import { resolveGlucoseUnit } from "@/lib/glucose";
+import { MGDL_PER_MMOL, resolveGlucoseUnit } from "@/lib/glucose";
 
 /**
  * v1.7.0 — `/insights/blood-glucose`.
@@ -25,8 +25,6 @@ import { resolveGlucoseUnit } from "@/lib/glucose";
  * exports. mg/dL-preference users are unaffected (scale = 1, integer
  * precision).
  */
-const MGDL_PER_MMOL = 18.0182;
-
 export default function InsightsBlutzuckerPage() {
   const { user } = useAuth();
   const { t } = useTranslations();

src/lib/glucose.ts

@@ -9,7 +9,15 @@
 
 export type GlucoseUnit = "mg/dL" | "mmol/L";
 
-const MGDL_PER_MMOL = 18.0182;
+/**
+ * mg/dL per mmol/L, per the DGIM / DDG S3 guideline. Exported because it is
+ * the one factor for this conversion in the tree: the inbound alias resolver
+ * normalises a mmol/L cell with it, the glucose insight page scales its chart
+ * by its reciprocal, and the helpers below round with it. Two nearly-equal
+ * copies of a health conversion is how the same reading comes out as two
+ * different numbers on two screens.
+ */
+export const MGDL_PER_MMOL = 18.0182;
 
 export function mgdlToMmol(mgdl: number): number {
   return Math.round((mgdl / MGDL_PER_MMOL) * 10) / 10;

src/lib/measurements/__tests__/unit-aliases.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect } from "vitest";
 
 import { resolveToCanonicalUnit } from "../unit-aliases";
+import { MGDL_PER_MMOL, mgdlToMmol } from "@/lib/glucose";
 
 describe("resolveToCanonicalUnit", () => {
   it("passes the canonical unit through case-insensitively", () => {
@@ -53,10 +54,17 @@ describe("resolveToCanonicalUnit", () => {
     });
   });
 
-  it("converts glucose mmol/L to canonical mg/dL", () => {
+  it("converts glucose mmol/L to canonical mg/dL on the app's one factor", () => {
+    // This used to assert 18.016 while every display path divided by
+    // 18.0182, so a reading imported as mmol/L and read back as mmol/L did
+    // not return the number it went in as. Both ends take the same factor
+    // now, and the assertion names it rather than repeating a literal —
+    // pinning the constant twice is how the two drifted apart.
     const out = resolveToCanonicalUnit("BLOOD_GLUCOSE", 5.3, "mmol/L");
     expect(out?.unit).toBe("mg/dL");
-    expect(out?.value).toBeCloseTo(5.3 * 18.016, 3);
+    expect(out?.value).toBeCloseTo(5.3 * MGDL_PER_MMOL, 3);
+    // The round trip a mmol/L reader actually experiences.
+    expect(mgdlToMmol(out?.value ?? 0)).toBe(5.3);
   });
 
   it("refuses an unrecognised or type-mismatched unit (never mis-stores)", () => {

src/lib/measurements/unit-aliases.ts

@@ -17,9 +17,7 @@
  * Pure + side-effect-free; safe to import from an API route or a script.
  */
 import { getUnitForType } from "@/lib/validations/measurement";
-
-/** Glucose mmol/L → mg/dL. */
-const GLUCOSE_MMOL_TO_MGDL = 18.016;
+import { MGDL_PER_MMOL } from "@/lib/glucose";
 /** Exact pound → kilogram (1 lb = 0.45359237 kg). */
 const LB_TO_KG = 0.45359237;
 /** Exact inch → centimetre (1 in = 2.54 cm). */
@@ -87,7 +85,7 @@ export function resolveToCanonicalUnit(
 
   // Glucose: mmol/L → mg/dL.
   if (type === "BLOOD_GLUCOSE" && MMOL_ALIASES.has(lower)) {
-    return { value: value * GLUCOSE_MMOL_TO_MGDL, unit: canonical };
+    return { value: value * MGDL_PER_MMOL, unit: canonical };
   }
 
   // Body mass: lb → kg.