refactor(medications): share the next-intake day label between card and table

The card and the table carried byte-identical copies of the day-label
arithmetic. Two copies is how they drift, and neither was reachable from a
test, which is why the double conversion in both survived review.

Lift it into resolveNextDueDayLabel, which returns a discriminated label the
callers map onto their own strings, and cover the case the old code could
not get right: a browser zone that differs from the profile zone. Those
tests pin the host zone explicitly — the defect was invisible whenever host
and profile agree, which is the only configuration a UTC-only suite runs in.

Also pin that the window-status verdict is identical from every host zone,
including one whose spring-forward gap swallows the profile wall clock.

src/components/medications/medication-card.tsx

@@ -14,7 +14,7 @@ import { getDateTimeFormat } from "@/lib/intl/formatter-cache";
 import { getMedicationCategoryLabel } from "@/lib/medications/category-label";
 import { formatDose } from "@/lib/medications/format-dose";
 import { reduceCurrentWindowStatus } from "@/lib/medications/window-status";
-import { localDayIndex, wallClockInTz } from "@/lib/tz/wall-clock";
+import { resolveNextDueDayLabel } from "@/lib/medications/next-due-day-label";
 import { resolveDisplayedSlotInstant } from "@/components/medications/card-parts/displayed-slot-instant";
 import {
   estimateDailyDoseCount,
@@ -241,7 +241,6 @@ export function MedicationCard({
       a.windowEnd.localeCompare(b.windowEnd),
   );
   const now = new Date();
-  const nowParts = wallClockInTz(now, userTz);
   // v1.8.4 — the next-due instant comes from the server (`nextDueAt`,
   // computed by the canonical recurrence engine anchored on the last
   // intake). The day label below derives from it; the window-range /
@@ -417,27 +416,15 @@ export function MedicationCard({
           // Format day label relative to today
           let dayLabel = "";
           if (nextAt) {
-            // Both clocks are read in the PROFILE timezone, and the day
-            // distance is a calendar-day difference — not an hour count —
-            // so a 23 h / 25 h transition day cannot round "tomorrow" into
-            // "today". `dateWithWeekdaySmart` receives the RAW instant: the
-            // formatter applies `userTz` itself, and feeding it a
-            // zone-shifted Date applied the offset a second time, which
-            // rendered the wrong calendar day for anyone whose browser zone
-            // differed from their profile zone.
-            const nextInstant = new Date(nextAt);
-            const nextParts = wallClockInTz(nextInstant, userTz);
-            const dayDelta = localDayIndex(nextParts) - localDayIndex(nowParts);
-
-            if (dayDelta === 0) {
-              dayLabel = t("medications.today");
-            } else if (dayDelta === 1) {
-              dayLabel = t("medications.tomorrow");
-            } else if (dayDelta <= 5) {
-              dayLabel = weekdayLabel(nextParts.weekday);
-            } else {
-              dayLabel = fmt.dateWithWeekdaySmart(nextInstant);
-            }
+            const label = resolveNextDueDayLabel(new Date(nextAt), now, userTz);
+            dayLabel =
+              label.kind === "today"
+                ? t("medications.today")
+                : label.kind === "tomorrow"
+                  ? t("medications.tomorrow")
+                  : label.kind === "weekday"
+                    ? weekdayLabel(label.weekday)
+                    : fmt.dateWithWeekdaySmart(label.instant);
           }
 
           // v1.16.1 — the TIME is the canonical next-due instant (the

src/components/medications/medication-table.tsx

@@ -29,7 +29,7 @@ import { resolveDisplayedSlotInstant } from "@/components/medications/card-parts
 import { estimateRunwayDays } from "@/components/medications/detail/supply-runway";
 import { useMedicationIntake } from "@/components/medications/use-medication-intake";
 import { reduceCurrentWindowStatus } from "@/lib/medications/window-status";
-import { localDayIndex, wallClockInTz } from "@/lib/tz/wall-clock";
+import { resolveNextDueDayLabel } from "@/lib/medications/next-due-day-label";
 import { formatTime } from "@/lib/format";
 import { formatTimeWindowRange } from "@/lib/time-window-format";
 import { useTranslations, useFormatters } from "@/lib/i18n/context";
@@ -418,7 +418,6 @@ function MedicationTableRowItem({
       a.windowEnd.localeCompare(b.windowEnd),
   );
   const now = new Date();
-  const nowParts = wallClockInTz(now, userTz);
   const nextDueMs = medication.nextDueAt
     ? new Date(medication.nextDueAt).getTime()
     : NaN;
@@ -533,22 +532,15 @@ function MedicationTableRowItem({
         </span>
       );
     } else if (nextAt) {
-      // Same contract as the card: both clocks read in the PROFILE
-      // timezone, a calendar-day distance rather than an hour count, and
-      // the RAW instant handed to `dateWithWeekdaySmart` — the formatter
-      // applies `userTz` itself, so passing a zone-shifted Date converted
-      // twice and could render the wrong calendar day.
-      const nextInstant = new Date(nextAt);
-      const nextParts = wallClockInTz(nextInstant, userTz);
-      const dayDelta = localDayIndex(nextParts) - localDayIndex(nowParts);
+      const label = resolveNextDueDayLabel(new Date(nextAt), now, userTz);
       const dayLabel =
-        dayDelta === 0
+        label.kind === "today"
           ? t("medications.today")
-          : dayDelta === 1
+          : label.kind === "tomorrow"
             ? t("medications.tomorrow")
-            : dayDelta <= 5
-              ? weekdayLabel(nextParts.weekday)
-              : fmt.dateWithWeekdaySmart(nextInstant);
+            : label.kind === "weekday"
+              ? weekdayLabel(label.weekday)
+              : fmt.dateWithWeekdaySmart(label.instant);
       nextCell = (
         <span>
           {dayLabel}, {formatTime(new Date(nextAt).toISOString())}