fix(frontend): fix off-by-one date shown in the backfill horizon picker

daysAgoToISODate built the date at local midnight but formatted it with
toISOString(), which converts to UTC first -- in any timezone ahead of
UTC (e.g. Europe/Paris), local midnight is still the previous day in
UTC, so the picker displayed a date one day earlier than the one that
was actually selected/stored. Format from the Date's local
year/month/day components instead of going through UTC.

frontend/src/pages/Profile.tsx

@@ -31,6 +31,54 @@ function NumberField({
   );
 }
 
+// Renders/edits BackfillHorizonDays as a calendar date (the day sync should
+// backfill through) rather than a raw day count, which is easier to reason
+// about than counting days back -- the stored value is still a day count
+// (what the backend actually consumes), just presented as its equivalent
+// date, recomputed against "today" on every render/change.
+function daysAgoToISODate(days: number): string {
+  const d = new Date();
+  d.setHours(0, 0, 0, 0);
+  d.setDate(d.getDate() - days);
+  // Deliberately not toISOString(): that formats in UTC, which in any
+  // timezone ahead of UTC shifts local midnight back to the previous day.
+  const y = d.getFullYear();
+  const m = String(d.getMonth() + 1).padStart(2, "0");
+  const day = String(d.getDate()).padStart(2, "0");
+  return `${y}-${m}-${day}`;
+}
+
+function isoDateToDaysAgo(iso: string): number {
+  const picked = new Date(iso + "T00:00:00");
+  const today = new Date();
+  today.setHours(0, 0, 0, 0);
+  const diffMs = today.getTime() - picked.getTime();
+  return Math.max(0, Math.round(diffMs / (24 * 60 * 60 * 1000)));
+}
+
+function BackfillHorizonField({
+  value,
+  onChange,
+}: {
+  value: number;
+  onChange: (v: number) => void;
+}) {
+  return (
+    <label>
+      Past sync (days)
+      <input
+        type="date"
+        value={daysAgoToISODate(value)}
+        max={daysAgoToISODate(0)}
+        onChange={(e) => {
+          if (e.target.value) onChange(isoDateToDaysAgo(e.target.value));
+        }}
+      />
+      <span className="field-hint">{value} day{value === 1 ? "" : "s"} ago</span>
+    </label>
+  );
+}
+
 // How long to wait after the last edit before actually saving, so typing a
 // name or a multi-digit number doesn't fire one request per keystroke --
 // only the last edit in a burst triggers a save, covering every field
@@ -162,8 +210,7 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
             onChange={(e) => set("GarminPassword", e.target.value)}
           />
         </label>
-        <NumberField
+        <BackfillHorizonField
-          label="Past sync (days)"
           value={profile.BackfillHorizonDays}
           onChange={(v) => set("BackfillHorizonDays", v)}
         />