refactor(tz): route coach/verdict/mood day-keys through userDayKey

The coach snapshot tzDayKey, the dashboard verdict dayKeyInTz, and the
mood-entry moodDateKey each carried their own Intl day-key formatter
(en-CA / en-CA / sv-SE). Delegate all three to the canonical userDayKey
so every day-bucket key stays byte-aligned and the locale decision
lives in one place. Behaviour-preserving.

src/lib/ai/coach/snapshot.ts

@@ -23,6 +23,7 @@ import {
   type CoachDataCluster,
 } from "@/lib/validations/coach-prefs";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/resolver";
+import { userDayKey } from "@/lib/tz/format";
 import { convertGlucose, resolveGlucoseUnit } from "@/lib/glucose";
 import {
   computeGlucoseClinicalMetrics,
@@ -223,15 +224,11 @@ function windowToDays(window: CoachScopeWindow): number {
  * "last night" answer has to use the user's clock, not UTC. Up to
  * v1.4.24 the day-key was UTC, so a 23:50 Pacific/Auckland reading
  * landed in the next UTC day's bucket and the Coach couldn't pair it
- * with the user's mental model.
+ * with the user's mental model. Delegates to the canonical
+ * `userDayKey` so every day-bucket surface stays byte-aligned.
  */
 function tzDayKey(date: Date, tz: string): string {
-  return new Intl.DateTimeFormat("en-CA", {
-    timeZone: tz,
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-  }).format(date);
+  return userDayKey(date, tz);
 }
 
 /**

src/lib/dashboard/verdict.ts

@@ -31,6 +31,7 @@ import {
   COACH_NUDGE_SLEEP_DEFICIT_MARGIN_H,
 } from "@/lib/jobs/coach-nudge-thresholds";
 import { buildWeightRangeFromHeight } from "@/lib/analytics/value-bands";
+import { userDayKey } from "@/lib/tz/format";
 import type { DashboardSnapshot } from "@/lib/dashboard/snapshot";
 
 const DAY_MS = 24 * 60 * 60 * 1000;
@@ -93,16 +94,6 @@ export interface DashboardVerdict {
   cta: DashboardVerdictCta | null;
 }
 
-/** YYYY-MM-DD key of `d`'s calendar day in `tz`. */
-function dayKeyInTz(d: Date, tz: string): string {
-  return new Intl.DateTimeFormat("en-CA", {
-    timeZone: tz,
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-  }).format(d);
-}
-
 /** HH:mm wall-clock of `d` in `tz`. */
 function hmInTz(d: Date, tz: string): string {
   const parts = new Intl.DateTimeFormat("en-GB", {
@@ -176,7 +167,7 @@ export function resolveDashboardVerdict(
     if (
       untilDue >= 0 &&
       untilDue <= DOSE_UPCOMING_WINDOW_MS &&
-      dayKeyInTz(new Date(dueMs), tz) === dayKeyInTz(now, tz)
+      userDayKey(new Date(dueMs), tz) === userDayKey(now, tz)
     ) {
       return {
         variant: "doseUpcoming",

src/lib/mood/date-key.ts

@@ -28,6 +28,8 @@
  * legacy date string (`effectiveMoodTz`).
  */
 
+import { userDayKey } from "@/lib/tz/format";
+
 export const DEFAULT_TIMEZONE = "Europe/Berlin";
 
 /**
@@ -35,14 +37,14 @@ export const DEFAULT_TIMEZONE = "Europe/Berlin";
  * timezone at write time. Used by `POST /api/mood-entries` and the
  * `PUT /api/mood-entries/:id` route when `moodLoggedAt` changes.
  *
- * The Swedish locale ("sv-SE") is the historical choice for ISO 8601
- * day output via `Intl.DateTimeFormat`; we keep it so the new path
- * produces byte-identical strings to the legacy `toBerlinDate()` for
- * Europe/Berlin users (the only path tested under the v1.4.24 suite).
+ * Delegates to the canonical `userDayKey` so the mood day-bucket key
+ * stays byte-identical to every other day-bucket surface. The ISO
+ * YYYY-MM-DD shape is unchanged from the legacy sv-SE formatter; the
+ * empty-tz fallback to Europe/Berlin is preserved here.
  */
 export function moodDateKey(date: Date, tz: string): string {
   const safe = tz && tz.length > 0 ? tz : DEFAULT_TIMEZONE;
-  return new Intl.DateTimeFormat("sv-SE", { timeZone: safe }).format(date);
+  return userDayKey(date, safe);
 }
 
 /**