feat(lint): refuse UTC day keys outside the time zone module

`instant.toISOString().slice(0, 10)` and parsing a key back as UTC
midnight are the idioms behind every day-boundary fix this year: the
evening west of UTC and the small hours east of it land on the
neighbouring day. A new rule, healthlog/no-utc-day-key, refuses them in
src/ outside src/lib/tz and tests, and refuses a disable directive for
itself that is not next-line or gives no reason.

The time zone module now names every legitimate form: userDayKey for a
person's day, dateOnlyKey / dateOnlyAtNoonUtc / dayKeyAsUtcMidnight for
date-only values and day labels, daysBetweenDateKeys / weekdayOfDateKey /
isCalendarDateKey / statedDateKey for key arithmetic and parsing.

Of the 133 sites the rule found, 84 are gone. Most moved onto a named
helper with no change in behaviour; these changed behaviour:

- cycle calendar, insights, Coach snapshot and prediction refresh start
  their lookbacks at the first instant of the user's day
- a model-transcribed date like "June 10, 2026" no longer lands a day
  early in a process east of UTC (documents and lab scans)
- a custom report range covers exactly the picked days in the profile
  zone instead of starting the evening before west of UTC
- date-of-birth pickers cap at the user's today, not the UTC one
- an allergy onset date is stored at noon UTC like other date-only values
- the report's cycle summary, the Coach's usual BP range and the intake
  projector's rollup recompute use the user's day
- vaccination dates render as dates instead of the previous evening

49 remain, each with its reason on the line above: 39 UTC by design (15
file name stamps, rollup and object keys, the UTC-day score engines) and
10 marked baseline. A guard test requires a classified reason on every
directive and holds the baseline to a ceiling that only goes down.

src/components/settings/account-section/index.tsx

@@ -33,7 +33,11 @@ import { TimeFormatSelect } from "@/components/settings/time-format-select";
 import { DateFormatSelect } from "@/components/settings/date-format-select";
 import { UnitPreferenceSelect } from "@/components/settings/unit-preference-select";
 import { GlucoseUnitSelect } from "@/components/settings/glucose-unit-select";
-import { detectBrowserTimezone, DEFAULT_TIMEZONE } from "@/lib/tz/format";
+import {
+  detectBrowserTimezone,
+  DEFAULT_TIMEZONE,
+  userDayKey,
+} from "@/lib/tz/format";
 import { apiFetchRaw, apiGet } from "@/lib/api/api-fetch";
 import {
   recentProofErrorMessage,
@@ -53,6 +57,7 @@ import {
 } from "./account-section-utils";
 import { AvatarSection } from "./avatar-section";
 import { SetupCard } from "./setup-card";
+import { dateOnlyKey } from "@/lib/tz/date-only";
 
 export { resolveInitialTimezone } from "./account-section-utils";
 
@@ -145,7 +150,7 @@ export function AccountSection() {
       email: user.email ?? "",
       height: heightAdapter.toDraft(user.heightCm),
       dateOfBirth: user.dateOfBirth
-        ? new Date(user.dateOfBirth).toISOString().slice(0, 10)
+        ? dateOnlyKey(new Date(user.dateOfBirth))
         : "",
       gender: user.gender ?? "",
       timezone: resolveInitialTimezone(user.timezone, detectBrowserTimezone()),
@@ -528,7 +533,7 @@ export function AccountSection() {
                   setDateOfBirth(next);
                   clearFieldError("dateOfBirth");
                 }}
-                max={new Date().toISOString().slice(0, 10)}
+                max={userDayKey(new Date(), detectBrowserTimezone())}
                 aria-invalid={fieldErrors.dateOfBirth ? true : undefined}
                 aria-describedby={
                   fieldErrors.dateOfBirth ? "dob-error" : undefined

src/components/settings/environment-section.tsx

@@ -227,7 +227,10 @@ export function EnvironmentSection() {
   // user can still type an earlier start, but pre-home days resolve to SKIP on
   // the server — to fill the deep past they add an explicit location period.
   const effectiveBackfillStart =
-    backfillStart || (home?.since ? userDayKey(new Date(home.since), tz) : "");
+    // `since` stays on the server's day for the home (a UTC day key in the
+    // environment module), so the default start matches what the worker
+    // compares against.
+    backfillStart || home?.since?.slice(0, 10) || "";
 
   return (
     <div className="space-y-6">

src/components/settings/export-section.tsx

@@ -246,6 +246,7 @@ function CsvCard({
         since: since || undefined,
         until: until || undefined,
       });
+      // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
       const stamp = new Date().toISOString().slice(0, 10);
       const filename = `${filenamePrefix}-${stamp}.csv`;
       await downloadFromUrl(`${endpoint}${query}`, filename);
@@ -329,6 +330,7 @@ function MedicationsCsvCard() {
         until: until || undefined,
         intake: includeIntake ? "true" : "false",
       });
+      // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
       const stamp = new Date().toISOString().slice(0, 10);
       await downloadFromUrl(
         `/api/export/medications${query}`,
@@ -447,6 +449,7 @@ function FullBackupCard() {
   }
 
   async function downloadOnce() {
+    // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
     const stamp = new Date().toISOString().slice(0, 10);
     if (encrypt) {
       const res = await throwIfReproofRequired(

src/components/settings/report-selection/report-visit-offer.tsx

@@ -25,6 +25,7 @@ import { Button } from "@/components/ui/button";
 import { useEncounters } from "@/hooks/use-encounters";
 import { useFormatters, useTranslations } from "@/lib/i18n/context";
 import type { EncounterKind } from "@/generated/prisma/client";
+import { detectBrowserTimezone, userDayKey } from "@/lib/tz/format";
 
 /**
  * How far ahead a booked visit is treated as the reason for this report.
@@ -36,9 +37,7 @@ export const REPORT_VISIT_HORIZON_DAYS = 14;
 
 /** The local `yyyy-MM-dd` a `DateField` takes, in the reader's own day. */
 function localDay(iso: string): string {
-  const at = new Date(iso);
-  const offset = at.getTimezoneOffset();
-  return new Date(at.getTime() - offset * 60_000).toISOString().slice(0, 10);
+  return userDayKey(new Date(iso), detectBrowserTimezone());
 }
 
 export function ReportVisitOffer({

src/components/settings/report-selection/selection-panel.tsx

@@ -66,6 +66,13 @@ import { orderLeaves } from "@/lib/report-selection/selection";
 
 import { ReportScopePicker } from "./report-scope-picker";
 import { ScopeSummary } from "./scope-summary";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
+import {
+  DEFAULT_TIMEZONE,
+  detectBrowserTimezone,
+  shiftDateKey,
+  validTimezoneOr,
+} from "@/lib/tz/format";
 
 type ExportFormat = "pdf" | "fhir" | "package";
 
@@ -78,6 +85,12 @@ class ReportRequestError extends Error {}
 export function HealthRecordExportPanel() {
   const { t, locale } = useTranslations();
   const { user } = useAuth();
+  // The report reads its window in the profile zone (the server's
+  // `reportTz`), so the picked days are cut there too.
+  const reportTz = validTimezoneOr(
+    user?.timezone ?? detectBrowserTimezone(),
+    DEFAULT_TIMEZONE,
+  );
   const queryClient = useQueryClient();
 
   const [format, setFormat] = useState<ExportFormat>(
@@ -155,8 +168,15 @@ export function HealthRecordExportPanel() {
     const range =
       customRange && startDate && endDate
         ? {
-            startDate: new Date(`${startDate}T00:00:00Z`).toISOString(),
-            endDate: new Date(`${endDate}T23:59:59Z`).toISOString(),
+            // The picked days are the user's days: from the first instant
+            // of the first to the last instant of the last, in their zone.
+            // UTC midnight started the window the previous evening west of
+            // UTC, and the report read it as one day more.
+            startDate: startOfLocalDayKey(startDate, reportTz).toISOString(),
+            endDate: new Date(
+              startOfLocalDayKey(shiftDateKey(endDate, 1), reportTz).getTime() -
+                1,
+            ).toISOString(),
           }
         : { days };
     const res = await throwIfReproofRequired(
@@ -190,6 +210,7 @@ export function HealthRecordExportPanel() {
     const url = URL.createObjectURL(blob);
     const a = document.createElement("a");
     a.href = url;
+    // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
     a.download = `healthlog-health-record-${new Date()
       .toISOString()
       .slice(0, 10)}.${ext}`;