Add configurable sync_lookback_days for Garmin Connect

Users can now set how many days back the first sync fetches. -1 syncs all
history back to 2010; any positive value sets a rolling window. Values
over 365 show a rate-limit warning in the UI. The default remains 30 days.

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

frontend/src/pages/ProfilePage.jsx

@@ -115,7 +115,7 @@ export default function ProfilePage() {
     queryKey: ['garmin-config'],
     queryFn: () => api.get('/garmin-sync/config').then(r => r.data),
   })
-  const [gcForm, setGcForm] = useState({ email: '', password: '', sync_enabled: true, sync_activities: true, sync_wellness: true })
+  const [gcForm, setGcForm] = useState({ email: '', password: '', sync_enabled: true, sync_activities: true, sync_wellness: true, sync_lookback_days: 30 })
   const [gcSaved, setGcSaved] = useState(false)
   const [gcError, setGcError] = useState('')
   const [gcSyncing, setGcSyncing] = useState(false)
@@ -127,6 +127,7 @@ export default function ProfilePage() {
         sync_enabled: garminConfig.sync_enabled,
         sync_activities: garminConfig.sync_activities,
         sync_wellness: garminConfig.sync_wellness,
+        sync_lookback_days: garminConfig.sync_lookback_days ?? 30,
       }))
     }
   }, [garminConfig])
@@ -145,7 +146,7 @@ export default function ProfilePage() {
     mutationFn: () => api.delete('/garmin-sync/config'),
     onSuccess: () => {
       refetchGarmin()
-      setGcForm({ email: '', password: '', sync_enabled: true, sync_activities: true, sync_wellness: true })
+      setGcForm({ email: '', password: '', sync_enabled: true, sync_activities: true, sync_wellness: true, sync_lookback_days: 30 })
     },
   })
   const triggerGarminSync = async () => {
@@ -345,6 +346,14 @@ export default function ProfilePage() {
               </label>
             ))}
           </div>
+
+          <Field label="Sync lookback days" hint="-1 syncs all available history (back to 2010). Leave at 30 for incremental syncs.">
+            <Input type="number" value={gcForm.sync_lookback_days} min={-1}
+              onChange={e => setGcForm(f => ({ ...f, sync_lookback_days: parseInt(e.target.value, 10) || 30 }))} />
+            {gcForm.sync_lookback_days > 365 && gcForm.sync_lookback_days !== -1 && (
+              <p className="text-yellow-400 text-xs mt-1">Warning: syncing more than 365 days at once may take a long time and could trigger Garmin rate limits.</p>
+            )}
+          </Field>
         </div>
 
         {gcError && <p className="text-red-400 text-xs">{gcError}</p>}