fix(charts): cut days in the account's zone when a mount passes none

Several chart mounts (the medications page, the coach chat, the
recovery section) pass no zone, and the chart then fell back to a fixed
Europe/Berlin, so those charts cut and labelled days in Berlin for
every user whatever their profile said.

The chart now falls back to the signed-in account's zone, and only
before that has loaded to the shared DEFAULT_TIMEZONE the server's own
resolver ends on. /api/auth/me reports the zone the server resolves:
the stored one, or the instance default when the stored one is
unusable, so the client cuts days where the server does.

Refs #1026

src/app/api/auth/me/route.ts

@@ -49,6 +49,10 @@ import { syncMfaEnrollCookie } from "@/lib/auth/mfa-enrollment";
 import { buildAvatarUrl } from "@/lib/avatar";
 import { decrypt } from "@/lib/crypto";
 import { prisma } from "@/lib/db";
+import {
+  isValidTimezone,
+  resolveServerDefaultTimezone,
+} from "@/lib/tz/resolver";
 import { isCycleEnabled } from "@/lib/cycle/gate";
 import {
   resolveModuleMap,
@@ -238,7 +242,12 @@ export const GET = apiHandler(async () => {
     heightCm: user.heightCm,
     dateOfBirth: user.dateOfBirth,
     gender: user.gender,
-    timezone: user.timezone,
+    // The zone the server cuts this account's days in: the stored zone, or
+    // the instance default when the stored one is unusable. Clients bucket
+    // by this value, so it must be the resolved one, not the raw column.
+    timezone: isValidTimezone(user.timezone)
+      ? user.timezone
+      : await resolveServerDefaultTimezone(),
     onboardingCompletedAt: user.onboardingCompletedAt,
     onboardingTourCompleted: user.onboardingTourCompleted,
     // v1.39 (C1) — the needs-based setup flow, published additively: