VO2 max carry-forward and sync lookback days fix

Show the most recently known VO2 max value on days where Garmin has
not produced a new estimate (it only updates after certain activities).
Fix the sync lookback days input resetting to the server value during
polling — the form now initialises from the server once on first load
and is not overwritten by background refetches.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/HealthPage.jsx

@@ -304,7 +304,7 @@ function NavArrow({ onClick, disabled, children }) {
   )
 }
 
-function DailySnapshot({ day, avg30, intradayHr, bodyBattery, bbHires, sleepStages, activities, onOlder, onNewer, hasOlder, hasNewer }) {
+function DailySnapshot({ day, avg30, intradayHr, bodyBattery, bbHires, sleepStages, activities, latestVo2max, onOlder, onNewer, hasOlder, hasNewer }) {
   if (!day) return (
     <div className="text-center py-10 text-gray-600">
       <p className="text-3xl mb-2">📊</p>
@@ -523,7 +523,7 @@ function DailySnapshot({ day, avg30, intradayHr, bodyBattery, bbHires, sleepStag
           <p className="text-xs text-gray-500 mb-1">VO2 Max</p>
           <div className="flex items-baseline gap-1">
             <span className="text-2xl font-bold text-blue-400">
-              {day.vo2max ? day.vo2max.toFixed(1) : '--'}
+              {(day.vo2max ?? latestVo2max) != null ? (day.vo2max ?? latestVo2max).toFixed(1) : '--'}
             </span>
           </div>
           {day.fitness_age && <p className="text-xs text-gray-500 mt-1">Fitness age {day.fitness_age}</p>}
@@ -673,6 +673,12 @@ export default function HealthPage() {
     return allDaysSorted.findIndex(d => d.date === selectedDay.date)
   }, [selectedDay, allDaysSorted])
 
+  // Most recent day with a VO2 max reading (Garmin only updates it after certain activities)
+  const latestVo2max = useMemo(() => {
+    const found = allDaysSorted.find(d => d.vo2max != null)
+    return found ? found.vo2max : null
+  }, [allDaysSorted])
+
   const { data: intradayData } = useQuery({
     queryKey: ['health-intraday', selectedDay?.date],
     queryFn: () => api.get('/health-metrics/intraday', { params: { date: selectedDay.date } }).then(r => r.data),
@@ -714,6 +720,7 @@ export default function HealthPage() {
         bbHires={intradayData?.body_battery_hires}
         sleepStages={intradayData?.sleep_stages}
         activities={dayActivities}
+        latestVo2max={latestVo2max}
         onOlder={goOlder}
         onNewer={goNewer}
         hasOlder={selectedIdx >= 0 && selectedIdx < allDaysSorted.length - 1}