feat(medications): thread timeZone into cadence + compliance helpers

The W19e pure cadence + compliance helpers used `Date.setHours`,
`getDay`, and `setHours(0, 0, 0, 0)` for every local-day boundary,
window-time application, and streak day-key. Those methods read the
host process's system time — every medication detail page on the
cadence route returned values relative to whatever zone the server
ran in (Europe/Berlin in prod, often UTC in CI), not the user's. A
Tokyo user reading the same data saw their 08:00 window land at
16:00 wall-clock and the day-of-week filter ran off the Berlin
weekday.

Thread an optional IANA `timeZone` argument through
`expandScheduleSlots`, `pairDoses`, `buildCadenceTimeline`,
`computeNextDose`, `missedDoses`, and `complianceChips`. Every local
operation now resolves through `Intl.DateTimeFormat`. When the
argument is omitted the helpers fall back to system-local — every
existing unit test (system-local by construction) keeps its
assertions green. The cadence route resolves the user's zone via
`resolveUserTimezone(user.id)` and forwards.

Regression tests pin the 08:00 wall-clock contract across Berlin /
Tokyo / Los Angeles and check that a "Mondays only" schedule lands
on each zone's local Monday.

Fix-O / code-H2.

src/app/api/medications/[id]/cadence/route.ts

@@ -26,6 +26,7 @@ import {
   computeNextDose,
 } from "@/lib/medications/scheduling/cadence";
 import { complianceChips } from "@/lib/medications/scheduling/compliance";
+import { resolveUserTimezone } from "@/lib/tz/resolver";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
@@ -76,12 +77,21 @@ export const GET = apiHandler(
     // chart disagree on the bi-weekly grid.
     const anchor = med.createdAt;
 
+    // v1.4.25 W21 Fix-O — resolve the user's IANA zone so the cadence
+    // helpers compute every local-day boundary, window-time application,
+    // and streak day-key in the user's clock instead of the host's
+    // system time. A Tokyo user reading the same medication gets the
+    // same chip values as a Berlin user — the host-relative day flip
+    // was a sneaky bug at the 08:00-Berlin / 16:00-Tokyo intersection.
+    const userTz = await resolveUserTimezone(user.id);
+
     const timeline = buildCadenceTimeline(
       med.schedules,
       events,
       asOf,
       windowDays,
       anchor,
+      userTz,
     );
     const chips = complianceChips(
       med.schedules,
@@ -89,8 +99,9 @@ export const GET = apiHandler(
       asOf,
       windowDays,
       anchor,
+      userTz,
     );
-    const next = computeNextDose(med.schedules, asOf, 14, anchor);
+    const next = computeNextDose(med.schedules, asOf, 14, anchor, userTz);
 
     annotate({
       action: {