Schedule backups in local time instead of UTC (0.66.0)

Schedule times were UTC, so a nightly 03:00 backup had to be entered as
01:00 or 02:00 depending on the season, and moved by an hour twice a year.

Each schedule now has an IANA time zone, preset in the form to the
browser's (e.g. Europe/Berlin), and its hour/minute/weekday are wall-clock
time there. compute_next_run builds each candidate day from its date in
that zone rather than adding 24 hours, so the time stays put across
daylight saving changes; a time skipped by the spring change runs an hour
later that day. next_run is still stored as naive UTC.

- Existing schedules default to UTC and keep their run times until edited.
- The API validates the zone and hour/minute/weekday ranges (400) instead
  of silently wrapping them.
- tzdata is a dependency: the slim base image has no system tz database.

Tests cover Berlin winter and summer time, both 2026 change days, the
skipped hour, a weekly run across the UTC date line, hourly at a +5:30
offset, the UTC fallback and the API checks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.65.0",
+  "version": "0.66.0",
   "type": "module",
   "scripts": {
     "dev": "vite",

frontend/src/api/schedules.ts

@@ -12,6 +12,8 @@ export interface BackupSchedule {
   hour: number;
   minute: number;
   weekday: number;
+  /** IANA zone the time is in, e.g. "Europe/Berlin". */
+  timezone: string;
   include_volumes: boolean;
   stop_first: boolean;
   keep: number;
@@ -34,6 +36,7 @@ export interface ScheduleInput {
   hour: number;
   minute: number;
   weekday: number;
+  timezone: string;
   include_volumes: boolean;
   stop_first: boolean;
   keep: number;

frontend/src/pages/Settings.tsx

@@ -77,8 +77,23 @@ export function Settings() {
 
 const WEEKDAYS = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
 
+/** The browser's zone: what "local time" means to whoever sets the schedule. */
+const BROWSER_TZ = Intl.DateTimeFormat().resolvedOptions().timeZone || "UTC";
+
+/** Every zone the browser knows (ES2022), or a short fallback list. */
+const TIME_ZONES: string[] = (() => {
+  const all = (
+    Intl as unknown as { supportedValuesOf?: (key: string) => string[] }
+  ).supportedValuesOf?.("timeZone");
+  const zones = all?.length
+    ? all
+    : ["Europe/Berlin", "Europe/Vienna", "Europe/Zurich", "Europe/London", "America/New_York"];
+  return Array.from(new Set(["UTC", BROWSER_TZ, ...zones]));
+})();
+
 function describeSchedule(s: BackupSchedule): string {
-  const t = `${String(s.hour).padStart(2, "0")}:${String(s.minute).padStart(2, "0")} UTC`;
+  const tz = s.timezone || "UTC";
+  const t = `${String(s.hour).padStart(2, "0")}:${String(s.minute).padStart(2, "0")} ${tz}`;
   if (s.frequency === "hourly") return `Hourly at :${String(s.minute).padStart(2, "0")}`;
   if (s.frequency === "weekly") return `Weekly · ${WEEKDAYS[s.weekday] ?? "?"} ${t}`;
   return `Daily · ${t}`;
@@ -312,6 +327,8 @@ function ScheduleForm({
     hour: schedule?.hour ?? 3,
     minute: schedule?.minute ?? 0,
     weekday: schedule?.weekday ?? 0,
+    // New schedules are in the browser's local time; existing ones keep theirs.
+    timezone: schedule?.timezone ?? BROWSER_TZ,
     include_volumes: schedule?.include_volumes ?? true,
     stop_first: schedule?.stop_first ?? true,
     keep: schedule?.keep ?? 7,
@@ -429,7 +446,7 @@ function ScheduleForm({
         )}
         {form.frequency !== "hourly" && (
           <label className="space-y-1">
-            <span className="text-xs font-medium text-slate-500">Hour (UTC)</span>
+            <span className="text-xs font-medium text-slate-500">Hour</span>
             <Input type="number" min={0} max={23} value={form.hour} onChange={(e) => set("hour", Number(e.target.value))} />
           </label>
         )}
@@ -437,6 +454,23 @@ function ScheduleForm({
           <span className="text-xs font-medium text-slate-500">Minute</span>
           <Input type="number" min={0} max={59} value={form.minute} onChange={(e) => set("minute", Number(e.target.value))} />
         </label>
+        <label className="space-y-1">
+          <span className="text-xs font-medium text-slate-500">Time zone</span>
+          <select
+            className={selectClass}
+            value={form.timezone}
+            onChange={(e) => set("timezone", e.target.value)}
+          >
+            {!TIME_ZONES.includes(form.timezone) && (
+              <option value={form.timezone}>{form.timezone}</option>
+            )}
+            {TIME_ZONES.map((z) => (
+              <option key={z} value={z}>
+                {z === BROWSER_TZ ? `${z} (this browser)` : z}
+              </option>
+            ))}
+          </select>
+        </label>
       </div>
       <div className="grid gap-3 sm:grid-cols-3">
         <label className="space-y-1">